Criterium - closed road circuit race, common form of bunch racing. Can be done in parks, or anywhere where motor vehicle access can be controlled, such as town centres in the evening.
In the UK, before the introduction of 7-day opening for shops/stores/DIY warehouses, etc., they were often run on industrial/commercial estates.
We've just had our local half-mile cycling circuit, at a city council sports centre, re-built for £160k and expect to be organising regular ones from now on.
Under British Schools Cycling Assoc. rules, distances should be as close as possible to:
U8s - 2 miles, U10 - 3 miles, U12 - 6.5m, U14 - 12.5m and U16 - 22m.
They are fast and furious as the kids get older and require good bike handling in the bunch and can be exciting to watch. You could run one for kids on a large car park if it's unused out of hours.