What are you trying to do?
Do you want to be sole signatories on her bank account so you can spend her money without checking with her?
Or do you want to protect her from being scammed by making sure that one of you also signs the cheque?
While she is still compos mentis, you should consider the power of attorney route.
From a practical point of view of paying bills, adding one of the children to the account to make it a joint account, might be the easiest.
Brendan