The family stand is just replacing the FE where you can go to have less grown men swearing at the team. (yeah right!) As per how it works now, you can take your kid in other areas. (bar the lower South in BMD which looks like it will be 18+)
The problem is for many if you've had a ST for 20 years, but your 8 year old has had a ST for 3 years, you have to wait for their window to open. Even if they use their age instead of number of years having a ST, you as the parent could see seats that you want, go to people that have had a far less tenure. That seems unfair whatever way you look at it.