I have a 10 and 13 yo, and here are their favorites:
Australia - used my nights in Sydney (in August), but also went to Cairns and Surfer's Paradise. LOTS of activities and fun for kids.
London - 13 y.o. loved it and has been angling to go back for a couple of
years now. Stayed at County Hall, but there are other 2 double bed options.
Hawaii - Honolulu Marriott and Maui. I don't think I'd use points in Honolulu as you can usually get a room for around $150 and there is no lounge anyways. I had a travel package that was expiring so I needed to use it.
Also really liked Hong Kong and Beijing, but that may not be everyone's cup of tea.
Lower on the list:
San Juan
Prague
Berlin
Amsterdam
I prefer to use my points at hotels with a lounge, which the above all do apart from Honolulu.