... Also, we do not charge the parents extra for this service so I understand it's not for "reward" as with taxis. ...
Sorry, but I believe your understanding is incorrect. Neither you nor your passengers are currently insured as the drop off / collection service is part of what you charge for. Your statement that you would like to "get a vehicle for the creche with all the correct insurances", implies you recognise this as well.
If you did not include the drop off / collection service, how many of your current clients, if any, would use your service? Another way of looking at it is to answer the question "How many, if any, of these children would be travelling in your car if you did not run a creche?"
"Standard" motor insurance covers the vehicle's use for "social, domestic and pleasure purposes". Which of these three categories do you believe the drop off / collection service fits into?
Most "standard" policies exclude the use of the vehicle for or in connection with "any business, trade or profession" as well as for "the carriage of goods and samples" and "for hire or reward".
Until such time as you get specific confirmation from your underwriters (NOT the broker), I would strongly encourage you to stop driving these children around uninsured.
I would suggest you start your search with the underwriters who provide your creche insurance