My 6yo dd is a slim too. She's in a 6X slim but should be in a 7 slim by the fall. I actually got lucky & picked up a couple pairs of 7 slim at a garage sale last weekend! I like the jeans at Old Navy or JC Penney's typically. The Arizona brand (at Penney's) comes in slim also & are reasonably priced. They have the adjustable waist band.
I haven't bought any, but a friend loves Children's Place & I know her dd wears slim sizes too. HTH!