Tentative Itinerary for early May Trip. Madrid, Seville, Granada, Barcelona. Plans/changes?

Just do Madrid and Barcelona

I think the OP's itinerary makes perfect sense. Day trip to Segovia I can't comment on. Flying Granada to Barcelona is easy and cheap on Vueling.

It doesn't make sense to me to do only one day in Seville. Are you just going there to see the Alcazar and the Plaza de España? If you would prefer to see Granada, why not just go from Madrid to Granada and get to spend more time in either Madrid or Granada? (My choice would be Madrid). 10 days is really not sufficient time to see 4 cities.

You are sprinting from point to point with just enough time to "see there" instead of "being there".
Stay in Madrid four days with two days trips.
Train to Barcelona and stay for remainder of trip with day trips.
Both locations offer huge menus of experiences to sample.
Slow down and enjoy the time together.

With respect, in a vacation of 9-10 days one does not have time to explore any city in detail. Perhaps the OP is looking to enjoy the diversity of experiences available in Spain and identify locations for more leisurely exploration in future. That's typically been the way things have happened for me.

This notion that short visits are worthless must mean the posters who suggest that hate cruises; all a cruise does is open one's eyes to diverse locations so one can explore them in further detail in future trips. But I've loved that means of travel.

I am planning my first visit to Spain as well.. originally including Barcelona, however modified plans once I learned about the drought conditions in that region. The drought is severe with requirement of water rationing including hotels. Posts from other travelers recommended visit Barcelona in the future when the locals aren't having this problem.
While I regret not being able to visit a city that seems amazing, I look forward to seeing Madrid, Cordoba, Granada & departing from Seville instead of Barcelona. I had purchased my flights a couple months ago, and yet the difference in the fare was minimal and somehow no change fees with the carrier.

I really don’t see the point of avoiding Barcelona because of the drought there yet travelling to another area that is also experiencing a severe drought.

Have you defined the must-sees in each stop? Looks like some places are fairly well defined by interest, but deciding on attractions in other cities (Madrid and Barcelona particularly) might clarify the itinerary.

Segovia - yes “worth it” if it interests you more than Madrid’s museums and sites. Look at Toledo as an alternative. Transport may be slightly easier. I bussed to Segovia, you can compare with train and decide.

Unlike many on this board, I’m not a big fan of Seville. I saw the Alcazar, Cathedral, Plaza de Espana, and Barrio Santa Cruz in one full, but not rushed day trip from Cordoba. Arrived about 10:15 and left around 5. (Skipped lunch as is typical for me) if you are all high energy as your itinerary suggests, your plan is do-able. Get the Alacazar tickets in advance to assure an entry time that works for you.

Fly to Barcelona. Montserrat is a standard day trip just outside Barcelona.

Get Alhambra tickets in advance.

Your trip is too quick and too many one night stays for me, but I traveled with my young adults at your ages and they would have enjoyed your plan - a taste of many places. Make sure Dad is on board!

Re Barcelona drought, I just returned. One sign in my hotel to turn off water tap. Other than that and this board, I would not have known of a drought. I guess the Magic Fountain is on hiatus, so that was another clue. My hotel pool was open. Certainly no restrictions that I encountered.