Dear Richard,
The questions you ask are the same as those I am asked by many parents. However, I warn parents that their teenage children will soon dump their fathers and grab the tiller for themselves.
It is best to join a club and take part in the races. This is because clubs will provide support boats whilst racing is in process. You might find this reassuring for your children.
A parent/child combination works well because the jib can be rigged and you would sail on the same handicap number as a single sail boat with one person. Two children together also works well.
A new Sprint costs about the same as a five year old Ford Mondeo.
Second hand boats are available. There is one for sale at my club for £1450, which is not advertised on the association website. PM me for further details.