hi, We have a 9 day (excluding travel days) trip planned in March. Going into Barcelona and leaving from Madrid. Family trip with teenagers (if that helps with recommendations). Our rough itinerary is
day 1-3 : Barcelona
Day 4-5 : Valencia
Day 6-7 : Seville
Day 8-9 : Madrid
1. Would this be too hectic and if so, any recommendations on which one we should skip - Valencia or Seville?
2. Is it possible to do Montserrat when we are in Barcelona, or would this require an additional day?

I imagine most people will say Valencia , since Seville is a must on most itineraries ( deservingly so)
Valencia is definitely worth a visit, a lovely place with great beaches., perhaps next time ! We visited both cities twice.

When in March? If your visit happens to coincide with Las Fallas in Valencia (March 15-19), that is a unique and very memorable experience. We've been twice. Lodging may be expensive at this point, though.

Ditto to what ms_go has told you about Valencia during Fallas. It's incredible, but also incredibly noisy and crowded, and hotels will be pricey. But I've been 3 times and it's a unique, highly memorable experience.

If you have access to Discovery Plus, there's a wonderful episode of "José Andrés and Family in Spain" that was filmed in Valencia during Fallas this past March, and he teaches a master class in how to prepare a proper paella.
I just did a free trail of Discovery Plus to watch all 6 episodes.

Seville, unless you think your daughters will enjoy this:
Those pictures were taken with an older phone so the prevasive dirt and the garbage does not show.
We also did not take the opportunity to stick our heads through a plywood mannequin dressed traditional Spanish attire.
Managed to save 2 Euros, plus the sight of the a half starved horse, standing in its own piss, waiting to drag a carriage through a busy road
too depressing.

the lineups and crowed of tourists ( us !) can be frustrating and a turn off.

Spending a few days in Seville during la SS was worth it, though.
Also, visiting overcrowded Hermitage was ,most likely , once in a lifetime opportunity.
Glad I “ endured “ both.

Thank you - appreciate the inputs. We will be in Spain right around Las Fallas, so looks like it might be Seville to avoid the crowds.

Seville is just magic. I found Valencia to be a bit ho-hum!