20 days in Spain with 6 & 9yo

What time of year? Andalucia is brutally hot during the summer. If that's when you're going, I'd consider somewhere along the northern coast (there would probably be some beach days, albeit cool ones). Or the Pyrenees.

We will be there June 19 to July 9 - thank you

I think the heat could be rough on young kids unless you basically just plunk yourselves down at a beach. I've taken a long vacation on which I had week after week of 85 to 93 F heat, and it was wearing even though Ii live in hot, humid Washington DC. I couldn't very well complain simce I chose the travel dates and set the itinerary. Thknking back to how I would have reacted during my childhood...

as acraven said the south starts to heat up especially Seville which would be very hot and uncomfortable for the young ones if you plan on sightseeing in the day

Thank you both for your insight!

At ages 6 and 9 the children won't remember much of the trip. They'll remember things you wouldn't think they'd remember and won't remember things you'd think they would. When we took our children they were 10 and 14. My daughter wanted to swim and my son wanted to play basketball. We camped most of that trip but tried to ensure each got to do some of what they wanted to do. Since you know what your children like to do and what they like, select some places especially for them. Once my son saw his first castle he wanted to visit them all. We also tried to minimize places children generally find boring; like art museums. You'll also need to compromise on restaurants you eat at too, since they will probably want to go to one of those U.S. fast food places we never eat at. On that trip we travelled in July on it was hot everywhere except the Basque country.

If you want to go to the south I'd consider going first - go from Barcelona to Seville on the first or second day, spend time there or elsewhere in the south, including the coast, then to Madrid, finishing in Barcelona. The reason for this is twofold - mid June is starting to get hot in the south but by July it is more likely to be unpleasant. Also I prefer to end my trip in the city I am flying out of - otherwise you would probably need to fly from Granada or Seville or Malaga the night before your flight home, requiring a not-all-that useful one night in Barcelona. End in Barcelona and you can stay your week there as planned and just get a cab or train to the airport from your hotel on departure day.

I did 15 days covering Granada/Nerja, Toledo and Barcelona last year with similar age kids. They loved the Albayzin and looking at the Alhambra in Granada - especially the fort, less into the palace. We found a ruined castle in Lanjarron that they still talk about wandering through. Nerja was a fun beach break. In Barcelona they especially liked Casa Batllo (they give you an interactive ipad guide that was a hit), the beach, and the cable car to Montjuic.

General kid-travel words of wisdom :
You can take them to museums and cathedrals - mine did well because if you expect your kids to participate and enjoy an activity for a reasonable amount of time they likely will. Especially if they know that the next thing will be ice cream or a park.

Food - Spanish food and restaurants are pretty kid friendly. Buy some proscuitto at home to introduce them to the idea of jamon (or buy Spanish jamon if you want, but it's hard to find and pricey, and proscuitto is similar enough for a kid to get the idea) - if they like this then no matter what else is on the menu they won't starve! And croquetas and paella are pretty popular.

Hotels - apartments are great for families with kids. A two bedroom is usually less than a mid-range quad hotel room and you have more space and a kitchen. I think barcelona has some restrictions on airbnb but there are apartment hotels and licensed agencies.

Heat - yes. It's hot. Drink lots of water and follow the siesta custom when possible. Go to your hotel or air conditioned museums during the heat of the day. It shouldn't derail your trip unless you are intent on a death march to sight after sight after sight each day (bad idea no matter the weather!). With time for breaks in parks or cafes, going back to the hotel for rest, and avoiding the hottest times you should be fine.

I would leave Andalusia for another time.
Travel by train ...
Madrid >> Cuenca >> Valencia >> Tarragona >> Barcelona

Ditto: From post above: In Barcelona they especially liked Casa Batllo (they give you an interactive ipad guide that was a hit), the beach, and the cable car to Montjuic.

I'm not sure a 6 and 9 year old would enjoy Casa Batllo but they might, our 11 year olds really really enjoyed seeing it because of the "crazy" architectural curves and the colorful decorations/mosaics. And Rick has a good free audio guide for Casa Batlo that I'm sure the 9 year old could follow.

We did not sit on the beach but walked along it, thrilling that this was the Mediterranean (make sure you look at maps before you go) and simply looking for rocks/shells. Mine also liked looking all around Las Ramblas. My 11 year old daughter was thrilled to be pick pocketed on that famous avenue....she had a used tissue in her pocket, and all of a sudden felt something...and then noticed it was gone. She'll probably remember that more than most of our other vacations! (And for all I know the tissue fell out somewhere along the way and was not pick pocketed at all.). Terrific that you are going on a great vacation with your family.