We have two 8-year old children, just come back last we went/tried:
1. Gardens by the Bay
2. Luge and skyride
3. USS
4. Adventure Cove WaterPark
5. Legoland - Theme Park and Water Park
6. iFly
Had a very happy time all over the above places.
Hotel:
1. Innotel Hotel - Location very convenient but bed is small
2. Legoland Hotel - Children enjoyed so much
3. Siloso Beach Resort @ Sentosa - Price is reasonable, good place but too wet