I think Cailin said it all well.
The one thing i do with our kids, well dd atm cause ds is to small (still im starting) is to explain why you dont want them to do that. Dont just say 'dont do it', say i dont want you to do that because. Its a learning experience for them and if they know the answers they will only benefit in the end. Im hard on dd but only cause i dont want a naughty girl when we go out, or at home. There character plays alot with how to disipline them to so only you know what buttons to push to make him listen.
Sounds like you and Simon need to have a little chat on parenting. He needs to realise that your the main parent that disiplines the kids so appreciate that and dont go against you in front of the kids. That only instills in them that dad is boss, kwim.

Parenting is bloody hard.