Allamanda at Laguna Phuket - they also do good suites with multi rooms.
 This is cool as you can have a lounge and set stuff up and they have a
lot more space.  Kata Thani/Kata Bhuri (latter is more kid friendly,
same place though - sister resort with shared facilities), good dives
right off the beach or longboat 5 mins.

If hotel units then Laguna Beach at Laguna Phuket is probably the best
place in Phuket for kids.  Great pool, water slide, loads of activities.
 Bit more expensive but not much.

Bear in mind unless the odd beach dive (as said Kata is ok) day dive
means DAY dive - 8 hours would be a decent average.  90% is boats away
from Phuket.  Child care in Phuket in my experience is very good.  I
have been there about 6 times in 2 years - live in Singapore - the Thais
love children - this can also be said for Bali which would be an
excellent alternative.