In Onfolk, you can run payroll for anyone eligible to be on payroll in the UK, including under 18s.
There are a few differences, here's what to be aware of:
No-one under the age of 13 can work
Under 18s cannot work full-time in England, they must work part-time
Under 16s do not pay National Insurance (though they do pay income tax)
To add an under 18 in Onfolk, simply invite them as usual using the Add new starter flow, entering their age and hours worked as appropriate.