ANSWER - There are a few ways this could be approached magically. The first would be looking at it from the perspective of a breaking spell. This would be severing ties between you and his family - the downside of course being that you may want your children to retain a relationship with their grandparents. If this is the case then you probably want to avoid breaking the relationship and the connection between you and it might be better to look at ways to reduce the influence their interfering is having on you. Something like a shielding spell may well work better, something to stop the negativity of the in-laws from actually reaching you in the first place. I do actually have a spell in mind and by personalising it, I think you may have some success with it.
Get a large bowl with a flat bottom and place a stone in the middle. Place a representation of yourselves and your children on the stone (poppet,plasticine figures or even something as simple as just your names written on a piece of paper) and fill the bowl with water so the stone is left mainly uncovered. Take a box of matches, light a match and visualise the flame to represent the interference you are receiving from the in-laws via your ex partner. Take the time to really establish the link in your mind and in terms of spell words, something like:
"Your words cannot preach
If they cannot reach
With peace I surround
As your negativity drowns."
Place the match in the water and push slowly toward the stone. The flame will of course splutter out. Repeat the symbolism for every different specific way they try to influence and manipulate you. Visualise their intent washed away and drowned in the water and yourself and your children safe from the sphere of your influence. Try and repeat regularly until you start to feel distanced from experiencing any negative feelings from their interference. Hold the visualisation in your mind when you are speaking to your ex partner, simply imagine everything that comes from the parents as being washed away in the water that surrounds the peace and safety that is you and your family.
Oh and tell this silly arse to get a backbone and stop acting as a mouthpiece for his parents. He's a grown man for goodness sake, tell him to start behaving like one.
Image http://www.sxc.hu/photo/1363589 (Xesko82)