S.T.
hi there, my kids (now grown) had one that still lives collecting dust in the garage. they LOVED it and my husband still enjoys taking it for a whirl every now and then! like you, i was concerned about the lack of exercise. however, my boys tend to be active and got plenty of boinging about anyway. scooters are not 'safe', but then nor are bicycles, ponies, trees, skateboards or boys at large. make 'em wear helmets, discuss safety, and if you see them being baboons, ground 'em.
i say go for it, and by the way, congrats on their fiscal responsibility! let them enjoy it.
:) khairete
S.