Pet sitting and babysitting are technically quite different, but oftentimes the two are combined into one fun-filled evening. It may be too late to speak up and charge more if high-maintenance pet sitting is involved, but for the next time, you can reconsider what you charge.
Asking a child to feed the dog a cup of food and fill the water bowl certainly isn't worthy of higher wages. However, very easily a night at home with the kids and their dog can turn into much more work than anticipated.
The dog gets into the pantry and into the cereal because the pantry door wasn't closed. Then gets sick all over the carpet from eating Cap'n Crunch. The cat uses the new Oriental rug as her scratch pad. Cleaning out the gerbil cage turns into a rodent hunt through the two-story house.
Then of course are the semi-normal pet sitting duties— maybe you're asked to take the dog on a walk, or clean out the litter box, or even change out the water in the fish tank. These may be "the child's chore" but essentially you're heavily involved. If you absolutely don't mind at all, then no problem. If you don't feel comfortable overseeing a cage cleaning for a red-eyed rodent or a flesh-eating fish, just tell the parents you'd rather the child do that chore with them, not you.
Another tip: if something dramatic happens because of a pet, always tell the parents—in detail. It may seem tempting to try to scrub the rug until it looks brand new, or run out and purchase a new class fish if the other one gets flushed down the toilet, but save yourself the trouble and stress. These are life events, and you can't fix everything that goes wrong. Of course you'll do your best to clean and repair, but the parents need to know exactly what went wrong and why.
If it becomes a theme that each time you babysit for a certain family their pet consumes as much time as one of the children, up your charge by a dollar an hour. If the pet causes a big mess or disruption on just one occasion, tell the parents exactly what happened and how you handled it, and they may pay you more that evening for having dealt with the problem so well. You may also decide that a certain family's pet isn't worth the hassle of babysitting for that family.