That playing on mum's back seems to be a typical Shetland thing? My other cross lambs don't do it at all....
I would agree that Shetland is best eaten as hogget (or even older), if they are mainly on grass they put on weight in their second summer rather than their first, and the legs make just the best roast ever (on the bone and whole that is). So yes, wait and castrate when they are ready... just make sure you keep an "accurate" date of their births... It is just a bother that they take up space in the lambing pens...