Would appreciate any info on the weather in April in the Costa de Sol area and/or Sienna. In particular will it be warm enough to spend time at beach/ pool. (We have young children - ages 4 & 6 - so a day at the beach is a welcome 'break' to chill out between excursions).

April in the south of Spain can go from warm (not hot) to fresh and rainy.
Sea water and not heated swiming pools will be too cold. Walking around or playing on the beach, maybe sunbathing should be fine.
Sienna and Tuscany will certanly be colder.

It might be hard to find a swimming pool around April. Maybe on the Costa del Sol, since it's for tourists but normally it's later in the season to open the pools.

Blackduff

Salandos,
I think blackduff is correct. You'll probably find the heated pools open only on the Costa del Sol or Costa de la Luz.
On our last visit to Marbella at Easter, the Hotel El Fuerte had its pool open but it was covered, and plenty of Northern European guests were out sunbathing. It was in the 80s that day. You never know. The El Fuerte group gives info about their pools and whether they're heated, outdoor or indoor, covered or not. and if there's a pool for kids.
www.fuertehoteles.com

For an actual weather report on Marbella, and a table with average monthly temperatures, look at the www.marbellas.com weatherpage