H.W.
I like the idea of bartering on rent. You could make it worth her while and say "$80 cash for the day, or $70 in barter on the rent." I know $80 might sound like a lot, but consider that you will have her child for at least 10 hours. (8 being the latest start and 5 being the earliest pickup--you said evening.) This would pan out to $8 an hour. I pay my babysitters about $10 an hour, so that's still less than the average here in Portland.