Hmm interesting idk why they don’t make an option for the vest to have an add on or not especially for youthAmazon has listings for add-on straps for life jackets. This only works if the chest straps are exposed at the back of the life jacket....there are some brands that are, some that aren't.
I’ve been looking and so far it seems if I add a leg strap to it it will pull that bottom strap on the vest downUnsure. I ran into the same situation with my bean pole of a son last year. We came up with a secure, home made version of a add-on strap. Luckily he grew into the jacket this year so no strap required.