I assume they will have to register with PRTB.
You could do their tax returns for them or , if you don't feel confident about that, employ an Accountant for at least the first year.
A Power of Attorney loses its effect if the Donor ( the person who gave the Power to someone else) is no longer compos mentis. Your grand parents should be making an Enduring Power of Attorney ( an entirely different animal to an ordinary Power of Attorney) so that when they are unable to make their own decisions that their nominated Attorney can do it for them. Citizens Information online has plenty of advice on this.
mf