Need to check...Numbers

Can I look after a 1 year old, a 2 year old, a 3 year old, a 6 year old and a sibling of one of these who's under 1? Total 5 children but 4 are under 3

Yes. As long as you are happy to, and feel you can easily cope.

you can have a maximum of 6 under the age of 8 at any one time.
It is usually 3 under the age of 5 , but you can make an exception for a sibling (usually 1 under 1 but again, you can make an exception)

I usually write a brief 'risk assessment' of all the children, what equipment I need/ have, brief outline of daily routine etc. I think you are also meant to inform all parents of children attending that day, who may be affected by you having 4. I usually write a brief note saying I'll have 4 children under 5 on X day, until one of the 4 dies not require care.

You can have 4 under-5s ...'If a childminder can demonstrate to parents and/or carers and Ofsted inspectors or their childminder agency that the individual needs of all the children are being met...'
So it is up to you, your RAs and the specific needs of each child.
https://assets.publishing.service.go...EWORK_2017.pdf Page 26