Flower Girls and Ring Bearer

My FH and I are going to pay for the flower girl and ring bearer outfits, as their parents are already responsible for purchasing their own bridesmaid/groomsmen attire, we just didn’t want to put all of the burden on them. My wedding is October 24, 2020 and I’m wondering when we should purchase these outfits. I’ve already decided what I want for them and have everything picked out, but I’m hesitant on buying anything because our flower girls and ring bearer are young and I’m afraid they’ll have a growth spurt before then. When do you guys think we should buy them? Also, if we buy them soon should we size up or go with their current sizes?