He is what you call a troll. Someone who posts things online for the sole purpose of angering and upsetting people. He probably doesn't believe half the shit that he's typing, but is having the time of his life seeing everyone else get so worked up over it.

People like him are best ignored. For their words hold so little value that the second he's proven wrong or doesn't know what to say, he starts spouting obscenities to cover up his lack of ability to argue like a "civil adult".

Sonny, go ahead, start making jokes about me now. That will only prove me right.

Now back on topic:

No one is completely right or wrong in this scenario.

Whoever had the idea for her to come with gifts and the kids simply made a very naive mistake. Chances are it was a joint decision. Nonetheless, the daughter needs to grow up and the LW and husband need to learn how to deal with this situation without trying to force anything.