Madrid/Sevilla/Barcelona-with a 7yr old

I went to the chocolate museum in Barcelona. It's mostly explanations (in English too) and pictures of the history and production. There are some chocolate sculptures too. There is a "kitchen" where you can have a hands-on experience (not active when I was there) but you'd have to contact them to find out more. Without that, it's kinda boring and I didn't like the free chocolate bar. She might like the Magic Fountain show - it's about 30 minutes, begins when it gets dark. If there's a 4 Guitars concert, do go - especially if it's at the Basílica de Santa Maria del Pi (much more intimate than the Palau de la Musica). There's a lot of comedy in the performance while they play serious music incredibly well.

Not in Sevilla, but as a day trip by train to see a horse show. Jerez has them at the Royal Equestrian School several times a week. I was told that if you go early, you can see the horses in the paddock before the show begins (reserved seating, buy in advance as they sell out). If you're there in the spring, you may see nesting storks. Besides being the sherry capital of the world, Jerez is also the flamenco capital. There's probably a museum in the city worth visiting. And there's a flamenco museum in Sevilla (haven't visited). I like the show at the Casa de Flamenco, it's early evening in a patio, an intimate setting so you're very close to the performers. There's also a horse show at the Royal Stables in Cordoba around 7 pm several times a week, maybe a miday show on weekends. It's shorter than the one in Jerez, and there's limited seating but closer to the performers. When I was there, seating was first come basis, so get there early for front row seats. If you go to Jerez, you could continue to Cadiz for the afternoon. Cordoba is worth a full day trip for the Mezquita and the Juderia.

I have not been myself, but I've heard mixed reviews on the chocolate museum. Instead I'd recommend:

Kid friendly day trips/activities for Barcelona: Tibidabo Amusement Park (oldest in Spain and quite whimsical with fantastic views), Dali Museum (quirky), Montserrat Hiking, Vall de Núria (fun in the Pyrenees), Roman Ruins of Tarraco, PortAventura World Park, CosmoCaixa (fun science museum)

Kid friendly day trips/activities for Madrid: Parque Warner Amusement Park, Roman Ruins of Segobriga, Ciudad Encantada geological site (Enchanted City), Alcazar of Segovia, Parque del Retiro boating

I agree the Royal Equestrian School in Jerez would make a fantastic day trip from Sevilla.

I've never been but there is a fashion museum in Madrid called Museo del Traje, from its website it seems to be more than only frocks (and the main hall is closed until March).

http://www.culturaydeporte.gob.es/mtraje/exposicion/permanente-cerrada.html

In Sevilla, Maria Luisa park including the boating lake around plaza de España might be of interest. Lots of child-friendly stuff there.

We did Madrid and Barcelona when our kids were 10 and 14. I know that's older but they still enjoyed many typical sights as the Sagrada Familia is so unusual, as well as Park Guell and Casa Batllo. Those are all whimsical, colorful and more attractive to kids. We did Tibidabo and it was good-gorgeous sunset views and a few classic attractions. It has been developed to be kinda like a typical amusement park (expensive and routine) so we just focused on those classic rides and spent maybe 2-3 hours there. Be aware getting there is a chore, and takes about 45 minutes. I would still do it again with kids, though. Also, our kids enjoyed walking through La Boqueria Market along the Ramblas walk to eat something fun: seafood on a stick (if she's adventurous!), lots of sweets, etc. A few hours at the beach might work too?

In Madrid they liked the Palace. We didn't stay long but it's interesting enough to walk through. I insist on a little art so we did RS tour of the Prado, again it was maybe 2 hrs tops, maybe you could go faster even. We didn't have time to enjoy the park much, that sounds like a great place for your daughter. My kids like to eat unusual things so the food is always an attraction: ham carvings from the enormous hocks hanging in windows, churros dipped in chocolate, etc. We liked Chocolateria San Gines a lot more than Chocolateria Valor. We went maybe mid-morning and the line wasn't too bad at Gines. Later in the day it was so long!

Haven't been to Seville yet so good luck there!

Also in Barcelona, make sure to try Rocambolesc, world's most interesting ice cream shop, from Michelin stared Roca Bros.. They have many unique and whimsical flavours on offer, usually based on pop culture. They are located in the heart of the Barrio Gotico.