There are MANY things to factor in here. Do you typically feed supper to your children that leave at 6pm? This child will need supper by 7pm, so if you don't normally provide it then you need to factor that meal into your costs for the extra hour.

Also I would have a completely separate contract for this hour of care noting many important details. The firm 7pm, the days it is applicable for (not every day of care) and I would also note changes of care during this extra hour. If you do not watch tv during regular daycare hours but plan to use it while cooking supper or after supper until the parents arrive make it known. I would just be clear that daycare ends at 6pm and then it is an hour of more relaxed care. I would be weary of keeping the daycare routine going for one child until 7pm (you'll be drained). From 6-7pm have some free play while you get supper on the table, eat, then tv or free play while you clean the kitchen kind of deal.

As mentioned make sure you have an escape clause for when it becomes too much, which is why I suggest a separate childcare contract in addition to your usual daycare.

If you are looking for more income due to empty spaces and do plan to take this route maybe consider offering care to the other families as well? Give them all a date night or night to do groceries or go to the gym. In which case you'd make more money to make it worth it. But again, have it as a separate care in addition to your home daycare that you can end when needed.