A few tips:
Stay away from jeans, cords or heavy canvas. They always seem to be a slimmer cut trouser.
I used to look for trousers that were wider in the hip. I loved my DS in soft jersey joggers, they always seemed perfect for littlies (I have an intense dislike of denim and heavy fabrics on little ones). Another good trick is to buy a bigger size in a crop trouser, works out to be a good purchase, my DS is still wearing some crop trousers I bought for him 3 years ago
(he wore them the first year as fuill lenght trousers over cloth, the next year as crops and this year they fit him as boardies!)
Tescos clothing is generously sized and washes and wears well