Thanks mamas!
I found great jeans at baby gap. I looked in other stores as well which were suggested by you moms(like JCPenny, Target ) but all these stores had jeans with adjustable waist bands starting for ages 2 and up. Baby gap had them for sizes 12-18 months and 18-24 months.
Updated:
Old navy has great jeans too for a lower price. I think I am returning the ones I got at baby gap.