I would not be offended if a mother asked to exchange a child's gift... for any reason. I always try to tape gift receipts inside the birthday card. It is pretty much a stab in the dark at best when picking out a gift. If something is missing from the present, I would want to have it fixed so the child could enjoy the present.

(Have you tried using a tennis ball? My kids have this type of toy and it works with tennis balls, which we buy at the dollar store)

Tomorrow is always fresh, with no mistakes in it. — L.M. Montgomery