It would seem appropriate that if families choose to bring their children in as guests (which might be for any number of reasons including financial or anticipated low usage or perhaps indeed a selfish use of a loop-hole) they have, unfortunately, decided not to take up the implicit offer of membership for that child and should lose the right to become adult members later. One alternative (as suggested above) is for reduced child guest allowances (e.g. 10 visits a year and no access to sports lessons) and a full joining fee for those children that haven’t paid c.18 years of subs. There could be a vote on the two options above.
It is also unfair on those children that are full paying child members to lose out on various membership benefits (such as access to oversubscribed sports lessons) to other ‘guests children.