bed wetting and tummy ache common in kids with threadworms. Both ds and dd complained of sore tummy and both wet the bed but neither complained of itchy bum (though I did catch dd itching).
You need to treat the whole family - don't know how old your kids are but you can get stuff from the chemist (Ovex). Treat them once then repeat in 2 weeks time. It is quite difficult to get rid of them. Kids must be in underpants/knickers at night - wash their bums AS SOON AS they get up in the morning and wash their hands and under their nails. Ruthlessly change bed linen/pyjamas - lots of hoovering, cleaning door handles etc. etc. - basically, just what you want when you're 36 weeks pregnant. If it persists, you can go to doc and get Vermox which is a bit stronger. Don't forget that the threadworm cycle only exists because we re-infect ourselves.
It's really common - kids pick it up everywhere (especially sandpits).