Changing a diaper on a teddy bear, life sized baby doll or stuffed Elmo is nothing like changing the diaper on a real life, pooping baby. In order to replicate this activity without borrowing an actual baby, you could practice by stuffing an angry octopus into a small plastic bag. About 14 times. For one change. And if you think the squid ink is bad, well?
Most children do not like going to bed. Most babysitting jobs span the exact timeline of getting children into bed. This is not an accident by the employing parents. They may have in fact secured your babysitting services entirely to avoid putting their own children to bed, for one night. A refusal to go to bed by the child, coupled with the 114th reading of Good Night Moon will wear down even the most patient of parents. They've decided it's your turn. Good luck. Wear protection.
Parents will tell you that as long as the kids are safe, happy, and healthy (and put to bed on time, as per the last point), they aren't fussed if you make a mess doing arts and crafts, feeding the kids, or playing games/building forts. This is a lie. We all hate coming home to a messy house and if you need to leave Junior in front of the television or video game for half an hour before we get home, it's fine with most of us if it means all the couch cushions are back in their rightful spots.
Parents who tell you outright that they try to limit their children to only half an hour of television per night are most likely to be the ones who let their kids watch television non-stop all week and are trying to lower their child's average hours of viewing for that week, all in one night, courtesy of you. Good luck with that. Start building that fort.
Use the 'Well I'm just going to call your Mother and ask her' threats wisely. Kids catch on to this really quickly (just like the fake calls to Santa and the Easter Bunny). Practicing a fake one-way conversation with fake angry Mom is good practice for any babysitter. Do some role play with your friends.
It's not always a bad thing if the parents don't have the money to pay you on the spot when they get home. Most of us feel so guilty that we made you wait for your money that we top up even more the next day when we drop it off. And never say 'It's too much.' We know what the real cost is of watching our own kids. It's not too much. Reading GoodNight Moon one more time? Now that's 'too much'.