You can still persuade your dh that he should be concentrating on visiting his Mother on his own. He can't have the kind of conversations that he will need to have, whilst looking after a baby and a four year old. Tell him you don't want to confuse or upset your children until you are sure that they have changed/ are no longer abusive/ are not vilifying you in front of him.
Your dh should try phoning first, several times my dh has called his parents (as they had tried to contact him), only for them to run me/ him/ our ds down and remind him why he is nc!
Do you honestly and truly love your dh? Can you imagine not growing old with your dh? I went back several times after huge fall outs with in-laws because I realised that if I didn't let him try, especially when it came to our DC then he would resent me. I hated every single visit! There were always more nasty comments, more interfering and so much more abuse. But in the end this quote whilst not politically correct comes to mind ' give them enough rope and they will hang themselves '. Your dh knows what they are like, time away from speaking to them will not have changed who they are! They will repeats the same mistakes and relive the same patterns as soon as he visits them.
You have options, option 1; you can choose to take the higher ground, be polite but aloof when in their company. Concentrate on the children when you are there and don't say anything nasty to or about his family. This will drive them crazy and they will be right back to running you down/ being nasty/ abusive..... (If you choose this option then make sure to ask for open public places for your first meetings)
Option 2, you stay at home with your children and offer a shoulder to cry on/ a hug/ an ear to listen whenever he returns. Again not running them down, being kind, loving and reasonable (basically the opposite of his family).
Option 3- you stay at home whilst your dh takes your children to meet his family. You will struggle here, it is so difficult waiting at home wondering what they are saying to him/ wondering who is holding your baby/ wondering are your children being cared for/ hearing arguments. You risk demanding to know everything they said and behaving unreasonable and paranoid (btdt!). Or you put a welcoming smile on your face, big smiles and hugs for your children and a 'I hope that you had a lovely time' for your dh.
Unfortunately option 3 is your future if you and your dh split, every time you hand your children over you do so knowing that he could be going there. He won't even have to answer your questions about his family and how they are treating your children. You are also giving his family what they have wanted from the beginning, you out of his life! Is this what you really want?
From what we have been through, I know that it was incredibly hard for my dh to walk away from his parents, sibling and extended family (unfortunately they became flying monkeys so dh cut them out too). I have so much love and respect for him for doing this! No matter how abusive they are he loves them! I know he misses them to this day, I know the love and strength it took for him to defend me and our son and go no contact! He is the bravest man I know and I am so lucky to have him! I would give my family up for him, but it would be so hard and would absolutely devastate me....My dh has been through three lots of counselling since he went nc. It was absolutely the right thing to do, they had destroyed his self esteem and he felt like he truly was an awful son, he believed the awful things they said about him. To the point of being suicidal! Try not to be too hard on your dh, no matter what has gone on he is obviously a good and loving man! He stood up to his family for you! That is amazing! He must be head over heels in love with you to be prepared to go nc with his family to protect you!