It's a tricky one, this.
If you are buying a pedigree (or in the case of collies, ISDS registered) pup, then 100% agree with Annie you want all relevant health checks and you should ask to see the evidence.
However, there is an argument to say that the less highly bred family mutt / farm collie is so much less likely to have any problems, you could be safer buying such a pup.
In an ideal world, of course both parents would have had all health checks anyway, but the fact is that the majority of unregistered dogs - especially mongrels - are so healthy and free from these kinds of defects that it simply doesn't occur to the owners to get these checks done.
All the cases of collie eye that I know about have been in dogs where one (or mostly both) parents were registered. It seems to not generally occur in the general population of farm collies, where two dogs of generally uncertain parentage (but both free of symptomatic defects and from parents and litters also free of symptomatic defects) are mated.
in the hills - there will be reputable breeders of working / trial collies whose lists you could go on (for instance Derek Scrimgeour up here, David Kennard in North Devon, to name two I know personally and can recommend) - but for a pet/light working dog, personally I'd go for a less highly-bred one. Best tactic is to find one locally you really like (in your case specifically checking temperament to suit life in a busy household with active children), find out how it was bred and see if the breeder is planning a similar litter. Would your farmer neighbour be able to point you at such a one, or at farms locally who produce the odd litter of this type of dog?