Understandable you don't want to issue ultimatums to your DH, but this does need to be stopped somehow. If they don't even talk to DH, what happens if your baby is not in the mood for skyping, having a bath/crying/feeding or whatever, how do they rearrange time slots etc? It is hard to imagine how this works.
If you appeared on screen saying hi! How's the weather over there etc (basically ignoring their sulk) would they actually not respond? If that's the case, try it. Perhaps DH will get fed up with this behaviour of his own accord if they are actually rude to his wife in front of him?
Could you make sure you, DH and baby are all genuinely out doing something nice together when next skype is due, then send a message saying, sorry we missed you here's a video of dgc to be going on with.
I think you need to give them an arena in which to cock-up time and again, so he starts to see.
I'd just keep popping up on their screen in the way of their view of dgc breezily saying things like O good you're speaking to DH again, thank goodness for that? Oh you aren't? Well this is a bit silly isn't it? How childish. Oh well, never mind. Just going go take baby off for his nappy change. Let us know when you feel like a proper chat, bye bye!