Back Up Care Requested by Parents

DROP OFF CARE
It's common practice around here for carer's to offer a vacancy as a drop-off place until it's filled but they don't usually have a contract for it.
Care is advertised on local forums/FB pages, often with a schedule of availability. If someone want to advance book a specific date or time slot, then they are normally expected to e-mail the funds in full at the time of reserving the place. Once fees are received, the carer will return e-mail confirmation of payment, dates/times booked/reserved, if a meal will be included which is subject to the time-slot booked, and that the advance payment is non-refundable because it's used to hold the place.
If someone doesn't want to advance pay, then they phone on the day and if the place is not taken by someone else, they they may drop off their child. Payment is in cash at drop off.

In both situations an additional $30 fee is charged at drop off which covers for late collection. It is made clear that late collection is not to be viewed as an option and that if a parent is more than 10 mins late, community services will be called as the child is considered abandoned.

BACK-UP CARE
I have in the past provided back up care to a family where grandparents were the primary carer and they needed someone if grandparents were not going to be available.
In this situation, they had a contract which was much like my normal contract except paid leave and stat pay didn't apply. Payment was required each day care was provided, in cash, at drop off time.
And care was only available if I didn't have my full quota on that day.

I was lucky as this was primarily for grandma to have appointments and situations which normally could be planned ahead for days when I wasn't at max capacity. i.e. the day when a PT'er wasn't in if those unwanted days had yet to be filled. It wasn't ever for the situation where the carer was sick and care was needed that very day. I usually had a couple of weeks notice.

RISKS
Based purely on my observations as I don't do drop-offs because of the risks....

1. Payment - You have to be firm enough to make sure the money is in your hand at drop off because if Mom "forgets" at pickup, you can easily find yourself working for free.
2. Late Collection - Protecting yourself from late collections - what are you going to do if they don't show up and get their child until 8pm?
3. Behaviour - for me, this is the big one - what if the drop off child is a complete brat or worse, aggressive and how will my client base who pay my bills every day feel about random kids and people dropping in and out?
4. Hygiene/Disease Control - making sure that a drop off kid isn't there because he's getting over a fever, diarrhoea or some other contagious condition which excluded him from this regular carer.
5. High Demand Children - What if you end up with a child who has never been away from Mom but she's dropped off because she has an appointment/needs to get shopping done and the child screams and clings to you the whole time.
5.