Taken together with the other issues (late collections, late payments, expecting you to supply basic essentials such as wipes, etc.) the overall impression is that mum is just taking advantage of you.
You can fix a lot of this in one go with an immediate contract review.
Add an extra hour to the booked time and charge for it, at antisocial hours rate if necessary. Bear in mind, many nurseries impose a minimum £20 surcharge on parents who are 5 minutes late. If she’s collecting late regularly, then she needs that time and must pay for it.
Late payments should attract an immediate £20 penalty fee. It’s difficult to add a daily charge or proper percentage for late payments. The ultimate deterrent, and well within your rights, is to refuse care until the bill is paid, and still charge for any days missed as a consequence. But this can sometimes backfire on you. One alternative is to increase her basic hourly rate, and then give a credit if she pays on time as an incentive. A lot of small businesses do this simple measure, but it seems surprisingly rare to find a childcare business following such a simple practice.
Introduce and apply charges for consumables when she doesn’t supply whatever is agreed in the contract.
Make sure your contract is clear and tight. Standard contract stationery, such as that sold by Pacey covers all these points so long as you fill it in diligently.