They do go free.
Your Idea is to let them into the ground without signing up to any scheme or allocating a ticket to them. How do you know where they're sitting, what the attendance is etc.
if you think taking 5 minutes to sign a kid up to JSB is hard work then you must be one fat lazy git.
Exactly, push them into the JSBS and they then have the kids details, date of birth, address etc for future marketing. Even if they don't have a season ticket they will be sent a sheet of vouchers in the post so a family of 4 can go for £22, so they can take a friend for a quid etc. Give them a CCFC branded free gift and get them down to the kids party, where I have seen with my own eyes little kids who don't start off that bothered about CCFC to running round with the other kids trying to get player autographs etc.
If for example it isn't a kid who's parent can take him every game for various reasons the vouchers are handy to get them to go to a couple of games when it's cheap and take a mate with them, they will get letters addressed to them (don't know about other people, but mine feels grown up when she gets a letter and loves it) about 3 times a year that will also remind them about CCFC / Sky Blue Sam.
That's as opposed to just waving kids in without knowing their actual age, who they are etc and just hoping they come back.