If a child were to be bit, it would definitely be more serious, though still very unlikely to be fatal.
Some species of Boiga like Boiga irregularis (brown tree snake) have caused serious (though I don't know about fatal) bites to children in Guam.
I have heard anecdotal remarks from one keeper who's friend suffered permanent damage in his hand (don't know the degree of this permanent damage, but it involved pain) from a bite from an adult mangrove on the knuckle.
The saving grace is that they have to get a real good chew on to give you a serious envenomation, so if you can get em off quickly, you shouldn't be in too much trouble.
In summary, a mangrove bite (or any rear-fanged colubrid) is best to be avoided...using proper handling techniques goes and protocols goes a long way. I have a big female who is very aggressive sometimes, but hook and tailing her and only handling when absolutely neccesary has spared me any bites.