I work in a nursery. I have up to 4 babies by myself. I have a 7 month old at the moment who will only sleep either in my arms, or in a cot on their belly. I am well aware that they should not be on their belly. I have expressed this to the parent, the parent just indicates that they will sleep no other way. I'm the only educator in the room. I simply can't just sit with one child in my arms. I have tried settling them in the cot on their back. I've tried settling in my arms and putting them on their back in the cot once they are well asleep. As soon as they touch the mattress the child becomes very very upset. Neither can I sit with a child asleep in my arms when there are many other tasks to see to. What am I to do?

Most babies will be rolling over by 4- 5 months…...                             
This is normal as babies grow & become stronger.
Many babies then prefer to sleep on their tummy or side.
This is not something that needs to be stopped. It is important to make the environment as safe as possible, now that baby is rolling

It is important that baby is not being wrapped.  Baby needs arms free & the space of a cot to roll well.
Once baby can roll both ways, place on back & let baby find their own position of comfort.
They can then be left to sleep on their tummy if this is their preferred position.

Ensure there is nothing loose or soft in baby’s sleep environment.
Always continue to place baby on back for sleep.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

What State are you from?
Do you have some guidelines about settling or a Child Health Nurse you can refer to about settling?
How does the parent settle the child at home for sleep?
Red Nose has the following information about settling.
https://rednose.org.au/article/settling-information-statement