No, they stay up fine. I think it's important to have the whole pant bit in the top of each leg. but I'll ask her whether there's anything else you need to do when she gets home from Scouts. She's got some she's worn for ages and they've been washes lots and still going strong.
She's been attacking her clothes with varying degrees of success all her life - and is currently making a ball gown out of bin bags at school!