My two favorite places to check for things to do and places to stay is TripAdvisor and European Destinations. I have been reading up on forums and blogs to see other people's experiences, and I even found a pretty neat website for seeing Barcelona with young children. Right now, I am working on Barcelona. Then I will move to Madrid once I have the details planned for Barcelona.
Here is a basic itinerary for now. Again, this is just what I have found so far and it may be altered as time goes on. After all, we won't be going until about the middle to end of August.
Day 1 - Arrive first thing in the morning. Check into hotel. Walk around the city to get a feel for our location. Find small things close by to do. If early enough, we could walk to Gothic Quarter - http://www.barcelona-tourist-guide.com/en/areas/gothic-quarter-barcelona.html.
Day 2 -
Parc Guell - http://www.parkguell.es/en/historia (as of right now, free of charge, approximately 2 hours)
La Sagrada Familia - http://www.sagradafamilia.cat/sf-eng/docs_serveis/informacio.php (13.50 euro, offers a guided tour - 1 hour long) - purchase tickets in advance.
Pablo Espanyol - http://www.poble-espanyol.com/en/visitors (11euro, offers kids activities and educational adventures)
Day 3 -
Start towards Montjuic (Mountain of Jews) - http://www.barcelonaturisme.com/Parc-de-Montjuic/_dHcEaVlbphwE-UMd45leEd5DlUsztdocdW4LHetpKP8
See either the castle or any museums in the area.
Head to Plaza Espana - http://www.tripadvisor.com/Travel-g187497-c113581/Barcelona:Spain:Plaza.De.Espana.html or http://en.wikipedia.org/wiki/Pla%C3%A7a_d'Espanya,_Barcelona
Walk down to Barconelleta for some beach time.
**Hop on, hop off bus is always an option - http://www.city-sightseeing.com/tours/spain/barcelona.htm#tourlandingposition (24.50 euro)
**Barcelona Card - http://www.barcelona-card.com/en/index.html
**Maybe Tapas Tours? http://www.tapastoursbarcelona.com/
I haven't done much searching for a hotel yet. I can't decide if I want to stay in a hotel that has a restaurant and offers room service or if I would rather find us an apartment and cook ourselves. For all lunches, we will be eating out and it seems we could experience the food. However, the convenience with the babies of room service has always been nice, but also expensive.