Gord Shell, you've received some good advice.
In most situations, parents have a Right of First Refusal in their agreement or court order. But, this isn't the case in all situations - like ours. In our case, my stepson's mother made it very clear that she will rely on her own resources to provide care for the child when she cannot do so herself on her custody time. (Yet she tried to insist that should the child's dad not be able to take care of him during his custody time, he would have to forfeit his access.) It was finally agreed that if either parent is unable to care for the child during their custody/access time, they will rely on their own resources (step-parents, family, friends, babysitters) to care for the child during that time. This makes the most sense given the 2hr distance between the child's 2 homes, and the fact that each parent has either a spouse or close family to help whenever needed. Naturally, we still attempt to swap weekends or vacations if possible.
I strongly suggest you get something in writing to protect your access rights. And if possible, include a Right to First Refusal in your agreement.
Good luck!