My toddlers all wore leggings until about 3/4 years old and they all seem to fit despite them being slim with very long legs :)
I’ve had success with George (Asda) they do good leggings which are much narrower on the waist and don’t fall down!
It’s hard isn’t it!!! We have resorted to dungarees and a few pairs of leggings that a friend took in at the waist
I find this hard with my Feb 22 baby. She still fits in some 9-12 due to having a small build but she is tall so thats the only reason we ever size up on trousers/leggings, the legs fit but the waistband is so loose. But tops in her correct size hang off her whereas 9-12 fits but might just be a little short 🤦♀️