I think it depends what you will see at the Hofburg, as there are a number of different things and each costs a separate amount.
If either of your kids are horse enthusiasts, take a look at seeing a performance or getting a tour at the Spanish Riding School.
I would suggest that perhaps the Imperial Treasury would be a little too slow for kids of those ages, but the entrance fee that includes the Silver Collection, Imperial Apartments, and the Sissi Museum is excellent. Skip the audio guide as it is too in-depth for kids (and perhaps most adults) and the kids might want just a quick trip through the Silver Collection, but the Imperial Apartments are still furnished as they were when the Kaiser lived there, which in my opinion is a bit of an improvement over Schoenbrunn. Finally, the Sissi museum is very well laid-out and it tends to be popular with people of all ages.
That being said, I just found out that there is a children's museum at Schoenbrunn, but you might as well take a look as obviously you would know better than I would if it might be a little too young for your kids.