I had this problem with DD. There is a product out there which wraps around the shoulder straps and velcros together. It used to be available from the Great Little Trading Co, but I've just looked and cannot find it - it was always out of stock anyway and kids can learn how to undo velcro quite easily.
I had previously been in contact with the local police station to get advice about this, and they suggested the following idea:-
to buy a clip buckle like those on rucksacks and a wide webbing strap, sew the bucle on with strong thick thread and then sew one side into one shoulder strap. The webbing was clipped together around both shoulder straps and tightened as much as possible (yes, it brought the straps to almost meet in the middle, but it worked).