They should open 2 accounts.

1:  Open one RESP that is in both of their names, and contribute all their Christmas and Birthday gift money.  Put it in low cost index mutual funds.  They will receive free government funds, and because the money is going into mutual funds no commissions are charged on the frequent small contributions.  When the money is withdrawn and used for school, the earnings are taxed in the kids hands.

2:  Open another informal trust investment account also in both of their names.  Put in only government money like Child Tax benefits, Canada Child Benefits etc.  There is a special tax rule that says that money earned in this account is taxed in the hands of the children, not the parent.   It is important that the account only contains these government benefits and no other gifts.  This can also go into low cost index mutual funds.