from my understanding, there is sand beside the pool at Hard Rock hotel, which most young kids would enjoy.
for Festive hotel, they have a loft that would be quite fun for elder kids (I would say at least 6 yrs old) whom not used to double level bed....
I think 5 days is good. one day at universal studio, one day for Sentosa, one day for city walk, one SG zoo, one day for casual walking/ enjoy hotel facilities.