Hi Ed,
Keep on the subject of the child. As mentioned, "If JW's can get so upset over children's birthday parties...what else?" We'll tell you.
If the son EVER wants to:
join a club, play a sport, go to college, go to dances, (among many things) it will be looked upon in the same manner. They are needless rules to keep control of the congregation. But it will strangle the life out of a childhood.
Make sure she understands this...the birthday conversation is JUST the beginning. Seriously.