As long as they're eating nutritiously, I wouldn't worry about their weight. Kids are the most natural of intuitive eaters, they will eat when they're hungry and stop when they're full until adults enforce their food wisdom upon them. Unless their blood tests are abnormal or the doctors show concern, I wouldn't worry much about it.
I knew plenty of scrawny kids (I was one myself, whom my parents sought to fatten with copious amounts of Ensure and hamburgers, without much success) who grew up into perfectly healthy adults. You're in a developed nation; food is plentiful. Your children will not starve, even if they're underweight.
F 28/5'4/100 lbs
"I'm not a psychopath, I'm a high-functioning sociopath; do your research."