Just a newby here, but I think your 7 and 4 year old on dirt bikes with a 16 year old is a legitimate topic of discussion and something you can likely put your foot down on and prevent if you think it too dangerous. It should be because of genuine concern for the kids, though, and not an attempt to drive a wedge between kids and W or even kids and OM because it would enmesh you back into what you've worked so hard to detach from. Also, your W could use it to drive a wedge between you and your kids (i.e., dad's the mean one, I'm nice, etc.).
Me: 46 W: 44 Married: 17 Together 21 D13; S10 BD: 03.03.15 (Not attracted to you) Almost 2 years trying, alone, to save marriage Status now: Divorced (effective 06.13.17)