How do I ensure that the guardians for my children have their living expenses covered?

You cannot leave something to one person for the benefit of another person. You cannot leave a sum of money to a legal guardian to fund the upbringing of the child. You either leave the sum to the guardian or you leave it to the child. Otherwise, somebody else would have to police the spending of that inheritance to ensure that it was only used for the child, and there are some grey areas like family vacations or home renovations that could be paid out of the child's inheritance "for the benefit of the child".
The correct approach is to leave the inheritance to the child. It would then be kept in trust and managed by a trustee (the Executor of your Will). The guardian can work with the trustee to release funds for the benefit of the child. With this approach, the needs of the child are addressed, but there is also accountability between the guardian and trustee. The funds can be used to help the child when they are still minors, but with a view to protecting the assets in the trust so there is something left for them when they become old enough to receive the trust outright.
The Executor of your Will would serve as the trustee, but there are two important clauses in your Will. The trust clause itself states that the Trustee
"may from time to time until he/she becomes absolutely entitled to all the capital of her share pay to or apply for the benefit of him/her the whole or such part of the net income derived from such share or the part thereof from time to time remaining in trust and such part or parts of the capital thereof as my Trustee in his or her absolute discretion deem advisable"
So the funds within the trust can be used for the benefit of the child as they are growing up. In addition there is a clause

"I authorize my Trustee to make any payments for any person under the age of majority or who is otherwise under a legal disability to a parent, guardian or committee of such person"

So the trustee can pass the funds onto the guardian if they determine that this is an appropriate thing to do. 
If you tried to set a fixed amount ahead of time, you would have to predict the situations that may arise. You don't know when the Will is going to come into effect and what needs the children may have. For example, the children may need to pay for advanced medical treatments that are unavailable in Canada, or legal fees, or tuition fees at a particular school, or sports or music activities. These could be paid for out of the trust if the trustee feels that they are in the best interests of the child. They can also release funds for the general maintenance of the children in consultation with the guardian, but there would be records of payments made and accounting that can be shared with the children.

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