Thank you for replying.
To reiterate, as an Ofsted registered childcare provider she should not be doing this. To be entirely accurate (hope this isn’t splitting hairs) she in breach of the EYFS regulations, rather than it being strictly "illegal".
You could complain directly to Ofsted. As you don’t have a child in her care, it’s not certain how they’d respond. They are likely to either get their compliance team to 'phone her, or else log it as a question to ask at her next routine inspection.
It might be best to inform the parent of the child concerned, since you know them. Perhaps the complaint would be better coming from them. The normal procedure would then be for the parent to discuss the matter with the CM in an attempt to reach a suitable arrangement. Failing that, the parent would use the CM's published complaints procedure, and then escalate that complaint to Ofsted if the CM still didn’t provide a satisfactory resolution. Ofsted will rarely do much unless and until the childcare provider's own complaints procedure has been exhausted (unless there is actual abuse taking place or a serious and immediate risk of harm.)
Do bear in mind the possible consequences. The CM may well decide that she cannot fulfil the regulatory requirements whilst continuing to work with this school and younger children. The result of that will be some families suddenly being left without childcare, or losing the continuity of having the same childminder throughout their childrens' preschool and school years. That’s a really big impact and would affect the parent you know personally. There’s no way of telling whether any of the families affected by these changes would thank you for it.
Other comments made about the school are valid and relevant to the situation, even if not directly relevant to your question. They do have a duty to enable other education/childcare providers to support children , and should be working with them in partnership, not putting obstacles in their way. Without going into too much detail on your pp, the issue of controlled access to the school is just as much a safeguarding issue as a caretaker being left to watch a buggy. Events on a sports field or school hall are somewhat different from having adults all over the entire building, and would be covered by an entirely different risk assessment.