I can think of nicer places to stay with your children than Sosua, the Den of Iniquity of the North Shore.
It is a great place to party and even families have had wonder vacations there, but please remember it is basically a sex and partytown and doesn't mind it at all. Cabarete is the same.
If you insist on beaches, you might be wise to investigate a place like Puerto Plata or even Nagua. Why, you ask?
Well for one thing there are some nice schools that are capable of educating your children, run by nuns that are reasonably strict.
Especially in Nagua, where the nuns run a tight ship in a town that really is no more than a beach community as well as an agricultural center. Puerto Plata also offers some Christian schools that I hear are good....by Christian, I mean one of the Prostestant varieties....
BTW, at those ages the kids will pick up Spanish like sponges, IF you expose them to it.....and they won't lose their English...
HB