Riding breech sizes for my niece?

so i have a 7 year old niece that i would really love to buy riding breeches for (jodhpurs). trouble is, is that i don'tt know her size in the breeches... her regular jean size is size 7... i tried to find a size chart on the INTERNET and its Absolutely no help what so ever... can any one help me PLEASE?! thank you so much! n_n