We went to Vanuatu last year with kids, 10 through to 16, Santo would probabaly get a bit boring for the non divers, there is plenty of good dive locations on the mainland, with reasonable places for the kids to enjoy snorkling as well.
We did all our dives with Tranquility Island resort, good variety of dives
Stayed at Le Lagon, they have kayaks, bar, sailboats,bar, Kids Club, bar,Golf Course, Pool, Bar, Restaurant, Bar
Local Beer is very addictive
Best Regards
Bruce