My kids (now 18 and 13) have been to both places at least a couple of times. They've enjoyed both, and I think either would be okay for kids.
Actually my kids probably enjoyed Chan Chich and Ek'Tun more than either Chaa Creek or Blancaneaux, mainly because both have great swimming pools. Chaa Creek is on the Macal Rvier and Blancaneaux is on the Privaison creek, but as it happens the times my kids were there the swimming wasn't too good.
Belize books and publications by Lan Sluder:
Belize First Magazine (http://www.belizefirst.com
Fodor's Belize & Guatemala Guide (new 4th edition coming this summer)
Adapter Kit: Belize (the first and only comprehensive guide to living, retiring, working and investing in Belize -- a best-seller on Amazon.com)
San Pedro Cool (the first and only visitor guide to Ambergris Caye)
Belize First Guide to Mainland Belize
UpClose Central America
Belize Book of Lists