30 days from Frankfurt to Barcelona with a 1 year old daughter and my parents in mid 60s

Top half looks fine, but notice you don't include Berlin - I would roll Dresden and Cesky Krumlov into 4 nights for Berlin. Bottom half might work better with some consolidation and simplification.

March 26: Fly into Frankfurt in the evening (one night in Frankfurt to recharge)
March 27: Train to Dresden (day 1)
March 28: Dresden (day 2)
March 29: Train to Prague (day 1)
March 30: Prague (day 2)
March 31: Prague (day 3)
April 1: Prague (day 4)
April 2: Bus to Cesky Krumlov (day 1)
April 3: Cesky Krumlov (day 2)
April 4: Train to Vienna (day 1)
April 5: Vienna (day 2)
April 6: Vienna (day 3)
April 7: Vienna (day 4)
April 8: Train to Venice (day 1)
April 9: Venice (day 2)
April 10: Venice (day 3)
April 11: Train to Rome (day 1)
April 12: Rome (day 2)
April 13: Rome (day 3)
April 14: Rome (day 4)
April 15: Fly to Nice (day 1)
April 16: Nice (day 2)
April 17: Nice (day 3)
April 18: Train to Provence (day 1)
April 19: Provence (day 2)
April 20: Provence (day 3)
April 21: Provence (day 4)
April 22: Train to Barcelona (day 1)
April 23: Barcelona (day 2)
April 24: Barcelona (day 3)
April 25: Barcelona (day 4)
April 26: Leaving Barcelona

I would not recommend a car except in France - not familiar with the areas you're going to, so I leave it to others if a car would be useful. The train will be more comfortable, especially for a toddler. Aside from the added difficulties of parking and city traffic, for the most part trains go at much faster speeds than you will in a car.

Those 1- and 2-nighters leave you little time for sightseeing. You will use 1/2 day or more for most of those moves. Have you looked at train schedules? Use bahn.de. I took a quick look - Frankfurt to Dresden is about 5.5 hours on the train (you still have to get to/from the station to/from the hotel, pack/unpack, check in/out) and you have to change trains. That leaves you a single day in Dresden for sightseeing. I didn't even look, but my guess is that Vienna to Venice is nearly a full day's travel, so you are left with only a single day to enjoy Venice. How quickly are the 5 of you going to be up, packed and ready to go in the morning? How much luggage are you taking with you (will you be able to move quickly on trams, metros and through train stations? Or will you need extra time?

Your current plan has consecutive nights as follows 1, 2, 4, 2, 5, 2, 2, 2, 2, 2, 4, 3. Presumably you are going to need to do laundry and shop for baby supplies.

You list Nice between Verona and Rome. That can't be right.

Thanks so much for your comment, much appreciated. Yes I did put Rome before Nice by mistake, it should be another way around. I totally agree with you, 2 days in those small cities is not worth packing and unpacking. What about this revised itinerary, I think I will ignore Italy for this trip.

March 26: Fly into Frankfurt in the evening (one night in Frankfurt to recharge)
March 27: Train to Berlin (day 1)
March 28: Berlin (day 2)
March 29: Berlin (day 3)
March 30: Berlin (day 4)
March 31: Train to Dresden (Day 1)
April 1: Dresden (day 2)
April 2: Dresden (day 3)
April 3: Train to Prague (day 1)
April 4: Prague (day 2)
April 5: Prague (day 3)
April 6: Bus to Cesky Krumlov (day 1)
April 7: Cesky Krumlov (day 2)
April 8: Train to Vienna (day 1)
April 9: Vienna (day 2)
April 10: Vienna (day 3)
April 11: Vienna (day 4)
April 12: Vienna (day 5)
April 13: Train to Budapest (day 1)
April 14: Budapest (day 2)
April 15: Budapest (day 3)
April 16: Budapest (day 4)
April 17: Fly to Toulouse (day 1)
April 18: Toulouse (day 2)
April 19: Carcassone (day 1)
April 20: Carcassone (day 2)
April 21: Carcassone (day 3)
April 22: Carcassone (day 4)
April 23: Barcelona (day 1)
April 24: Barcelona (day 2)
April 25: Barcelona (day 3)
April 26: Leaving Barcelona (day 4)

Cesky Krumlov is a 'bus to' destination and at 3 hours with a small child, I wonder if it's worth it.
Vienna is lovely but 5 days is more than most people would give it.
6 days for Toulouse and Carcassone? I'm wondering what the attraction is here. (I have been to both)
I say this as you follow up with only 3 days in Barcelona - you shouldn't be counting your departure day.
Sorry to see you discard Italy, a much more pleasant destination in April than northern Europe.

Well, even with revised itinerary, all that time in transit eats up almost a third of your time.

That's way too long in Carcassonne. Having been to Carcassonne for a couple weeks on business, and spent many years in the Nice area, I'd recommend going to Nice for the full time in France that time of year. The weather should be much better; there is plenty to do and see on the Riviera. I don't think five days in Vienna is too much. There is a lot to see and do, IMHO.

David makes a good point about weather. March-April in the north is liable to be cold, wet, windy, and gray. Snow is still a possibility in places like Berlin. While you may have high temps around 20, you are as likely to have highs that are under 10. Trees will be leafless, parks and gardens . . . not so nice.

If those are your dates, much better to go south. Italy in April is full spring. See more of Spain. Fly from Italy to Seville, Granada, or Malaga, visit Andalusia, then Madrid and end in Barcelona.

Chani - We don't want to be in cold areas as we would just want to pack for spring weather. Maybe I should just skip Berlin, just have 2 days at Dresden to brake the trip before heading to Prague, Vienna, Venice, Florence, Rome then fly to Southern Spain as per your suggestion. How many days should we spend in Spain in total?

Perhaps stay the first night at Frankfurt Airport and book an early flight to Prague for the next morning.
The train journey Frankfurt to Prague is 8-9 hours and needs connections.
Prague >> Vienna >> Venice >> Florence >> Rome >>
Either, fly to Nice >> Provence >> Barcelona.
Or, fly to Seville >> Madrid >> Barcelona.
With 30 nights to play with I wouldn't go beyond 8 cities.
Spring in Europe, you will need a reasonably thick sweater and sturdy waterproof jacket regardless.
Where would your parents like to go? Group meeting perhaps.

Thanks all so much for very useful and practical comments. I have finally firm my plan and will go with David's itinerary :) Now time to book flights and book accommodation on airbnb. If anyone of you have any recommendation on family accommodation (on budget) please feel free to share :)

We don't want to be in cold areas as we would just want to pack for spring weather.

Weather in Prague, Frankfurt, Dresden and even Vienna is going to be similar to Berlin. Winters in Melbourne are gentler than spring in northern Europe.

Exciting trip. You'll have a great time. Best wishes for all your planning and come back here when you have more questions. (RS forum members are the best).