And I thought you were gong to be honest and advise them to put their bank account into their child's name NOW, before it's too late!
The slippery road to perdition; if they can't quite
trust the kids, instead of hiding the money via proxy, you could always advise them to take up sailing instead - it's self-governing about how poor you ultimately become.
P.S. Thank goodness I haven't got a 'sister'. ;-)