Have them eat at Applebees, McDonalds, Burger King.  The riblet dinner at at
Applebees is 2100 calories and 130g of fat.

The  "Awesome Blossom" at Chili's is quite awesome,  Its loaded with 205
GRAMS OF FAT and 2700 CALORIES.

That'll start 'em.  Unfortunately, their arteries might close up too

Good luck

Go to the weightlifter sites. Trying to gain weight isn't as uncommon
as you might think. There is plenty of advice (try googling
hardgainer). What you need to gain weight is plenty of calories. This
does not mean gorging yourself on junk food, but eating lots of all
the staples pasta, rice, meat, fish, vegatables. Start at 3000
caloroies per day, and if that  doesn't work try 4000 etc. It will
however take some time and should of course be matched with a good
workout plan so that the weight isn't all put on as fat. If you find
your kids have difficulty eating enough try searching for tuna shake.
In the weightlifting community people prode themselves on how many
calories they can get into a tuna shake and still make it edible(ish).

Ray