Oahu has a great deal to offer kids 7 and 5. We took our kids to these places:

http://www.discoverycenterhawaii.org...fo/general.htm

http://www.honoluluzoo.org/

http://www.sealifeparkhawaii.com/

http://www.fonggarden.net/

You don't need to stay at the Kahala Hotel & Resort to use the beach there, which is ideal for young kids. The water is shallow for a long stretch and the waves are usually gentle.

http://www.kahalaresort.com/

I prefer Maui, but Oahu has the most to offer kids of these ages.