Multiply same service type when booking

I have clients who book in their kids, but can only book one kids cut, then have to create a separate booking for the other kid. We then have to put these together for checkout. As parents do this they usually book under their own log in. e.g. Dad wants to book in himself and two boys. He can book an Adult Cut and a Kids cut, but then has to go through the process again to book in an extra kids cut, rather than simply adding a second kids cut to his original booking.