Some of the breeds that I have heard families rave about for their protective and watchful nature in regards with kids are:
-Some rotties (I've heard really mixed reviews)
-Am pits from known breedings/lines
-Some GSDs (again, mixed reviews)
-Some Tibetan mastiffs, though the people I've heard that from have Asian lines, not the US lines which I hear are bred to be far more docile
-Bouvier des Flanders (sorry, probably massacred that spelling)
-Bull terrier (mixed reviews)
-Bull mastiff (mixed reviews)
I can say that our little ACD has protected my husband and myself on numerous occasions and in different situations (and once protected our best friend who was pet sitting her...the only person outside the family she has ever fully accepted). But, as you probably know from yours, so many heelers seem to gravitate towards adults, or only one person, seeing kids as either something to herd or something to ignore. Children can makes some ACDs very nervous. So I hear you on wanting a different breed for your kids.
I've heard other families talk about breeds that are great with their kids, but they either aren't protective of them, or are more geared to barking and warning than protecting, so I didn't include them.