Of course they go free, because they can't go by themselves

"Kids Go Free" coupon for LEGOLAND 

"Kids Go Free" coupon for LEGOLAND (reverse)

"Kids Go Free" coupon for LEGOLAND (Detail)

When I ordered some Christmas presents from LEGO in 2016, I received a coupon for a free kids' ticket to LEGOLAND, with a paid adult ticket.

The logic is pretty clear. Kids generally won't go to LEGOLAND on their own, a parent or parents will accompany them. And while the kids will be clamoring to go to LEGOLAND, the parents will often merely tolerate it. But once the kid sees this coupon for free admission they will be bugging their parent until either coupon expires or the parent relents. 

Given that the other spending associated with a park visit; food, parking, gifts, and souvenirs, will often exceed the cost of admission, this is a very smart piece of marketing.