I would've hoped you would have been inundated with replies given its probably a similar boat to a lot of us. Perhaps none of us have figured it out quite yet?!

Some suggestions for you though....

-Let the new coach be the primary coach for your son at the midweek training sessions. If he needs some constructive feedback, perhaps that can come from your colleague. If it something valid, and can be 

-Hold them to the same standard as the other children, both from a football perspective and a social perspective. If he's 'messing around' dont be any harsher on him than you would any other child. Equally, if he does something great and needs telling, dont be afraid to. 

-Dont continue the coaching in the car on the way home. Same can be said for overzealous parents. Take that time to switch back into parent mode, not as an opportunity to go all in on your captive (or captured!) audience. 

Full disclosure, Mrs P will often ask me 'Whats happened?!' when me and my son have come in from football training..... So I to am open to suggestions

I managed my son's team from u8-u11 and it was never really an issue. I did say to him he had to set an example and only pulled him up on stuff when he wasn't trying or paying attention, and messing about. The rest of the time I let him get on with it. We also had two coaches which helped. Good luck.

The reason I’m a coach is that I took the plunge and started managing my son’s team when it was created at U7’s, we’ll be U10’s this coming season. To be honest, initially I was quite him, but I soon learned the hard way to park that method of coaching and treat him like every other boy in the team. It’s really important to do this. 

if he has a bad game, I don’t immediately jump on him the second it’s just us in the car on the way home. I give it a day or two and try to suggest how he might improve if similar situations occurred again. I don’t sugar coat my assessment but I try to be constructive and encouraging. On the flip side if he’s had an absolute blinder, I’m the first sing praises to him. 

Most coaches kids feel the burden already, first to arrive, last to leave, help setting up, think they’ll never win a trophy because their parent doesn’t want to be accused of bias. I don’t agree with that last past, if my son has done well enough to earn Player of the Match or Trainer of the week then they get the award. Hope this helps. 

I’ve coached my eldest son since u9 (about to start u13) and my youngest and his for three seasons.

it’s never been an issue at all tbh and I think you’ll worry unnecessarily about it. Def been a blessing doing it and only strengthens the bond