Take Dream World if your kids are 5-12 — it is the closest thing Bangkok has to a full theme-park day, and the fake-snow room is a hit with children who have never seen snow. Buy the inclusive ride package; paying per ride adds up fast. Go on a weekday if you can — Thai school holidays and weekends double the queues, and most parks open around 10 am, so arriving at opening beats the crowds either way.
Skip Bangkok's theme parks entirely if Singapore or Dubai is also on your trip — Universal Studios and IMG Worlds outclass everything here, and Bangkok's strengths lie elsewhere: temples, food, and markets. Use park days in Bangkok only when travelling with children who need a break from sightseeing.
Pororo AquaPark is the underrated pick for toddlers: small, shallow, on top of a mall with food courts below, and easy to reach by BTS. For animal parks, see Bangkok safari and wildlife; for a full itinerary, see the Bangkok guide.