ExDP had two DC when I met him, we had them lots for several years. Contact then became sporadic and he wouldn't do anything to sort things out. Eventually he decided to walk away rather than have the DC stuck in the middle of disputes 
Unfortunately by this point we already had two DC so I was a bit stuck. I wanted to condemn him over walking away but was aware that may well cause him to walk away from our DC, too. His parents stood by his decision and ignored letters from his exW and her children asking them to keep in touch/have contact even if exDP wouldn't.
There was abuse in our relationship and he had very little to do with our DC together. He wanted the picture perfect family, but didn't want to do any of the parenting. The final straw was when the 2 DC and I had sickness bugs and were extremely poorly. DC1 had diarrhoea that leaked all over me and the bed so I cleaned her up and then left her settled but awake while I went to get cleaned up, being sick several times while I was there. I returned to her crying and DP nowhere to be found. DC2 was also crying with a leaky nappy. DP had left to stay at a friend's house as we were obviously unwell and he didn't want to disturb us in the morning...!
I began saving to leave as I knew he wouldn't leave. Just as I had everything in place, I discovered I was pregnant. I kept the baby but I still left him. He responded by going out drinking a lot and he refused to tell his mum what had happened between us. She treated me with contempt; I think she presumed I had been unfaithful and was pregnant by someone else. Eventually I had enough of her rudeness (repeatedly messaging me saying ExDP was a mess, didn't I care, how could I do this to him and the DC?) and the next time she asked what went wrong, I gave her a watered down version. It was still enough to show abuse, probable infidelity and him being so lax as to put the DC in danger, plus I said I'd lost respect for him over stopping seeing his other DC. MIL responded that relationships have blips, that I'd overreacted and needed to grow up and let DP get back to being an amazing father!
PIL met the baby once. Their son has walked away and they stand by him. My parents are no longer with us so DC actually ask about PIL more than they do about their dad but PIL want nothing to do with them unless via their son.
If my son has DC and becomes separated, no matter my love for him - I just cannot imagine abandoning my GC. Do in-laws always side with their DC, or have I just been unlucky?