Hi you only have to register if they have another job or they exceed the Tax & NI lower earnings contributions Tax £125.00pw NI £95.00pw double check these figures on the HMCE website or Business Link both really useful and if unsure give them a bell, you do however have to produce a payslip to your employee no matter how little they earn and get them to fill out a P46 you do not have to send this off until they reach the lower earnings threshold but get it signed and filed until they do.
Hope it's been of some help!!
Nat