Congratulations on your new baby! Of course you want the grandparents to be as thrilled and welcoming to your children as possible. You sound sad and frustrated by their negative responses.
How does your husband respond? We have some of the same issues w/ my in-laws and I've found the best response is none at all from me. I let DH be the bad guy. Thankfully, we are on the same page as far as parenting style & he completely acknowledges that they are HIS parents so he needs to talk to them. Have you discussed your concerns with your husband? How does he respond to their comments? Does he address any of these concerns with his parents?
Like you, I sometimes find that me simply not being there is the best solution. My in-laws live 20 minutes away; they see our kids at least twice a week, usually just w/ DH. My husband usually visits them when I am at work or otherwise busy, so I have an "excuse" for not going along. Added bonus is that DH has to handle any conflicts!
We've also found that sometimes we need to take a break from spending too much time with them. My MIL can be incredibly negative towards DS, who has some behavioral challenges & isn't as much "fun" for her to spend time with. Our older DD likes going out to eat, going shopping, and is willing to behave the way Grandma considers appropriate for the reward of getting to do those things. DS simply doesn't care. For a while, MIL was saying some incredibly negative and (IMO, of course) inappropriate things to and about DS. He was definitely hearing what she said; even worse, DD was hearing it and repeating it back to her brother. So we simply didn't let our kids be around them without one of us present until she stopped.
It will get easier as your children get older. MIL doesn't even comment about our current toddler's frequent nursing. She didn't ask to feed her at 2 months or expect us to leave her alone with them right away. She isn't pushing for overnight visits. With the older kids, she was doing all these things. So she's learned!
It's especially hard when the negativity is for something you KNOW is developmentally appropriate. Of course your 19 month old doesn't want to go to them. That's normal. But when there's already a strained relationship, it's not so easy to suggest that they sit on the floor and read her favorite book aloud until she warms up (or whatever would help your child), because they don't want to listen to YOUR suggestions. I've spent years hearing, "They behaved just fine until YOU got back". Of course they did - they feel safe now that Mom and Dad are there and can let out whatever pent up frustrations they kept inside while MIL was watching them. But of course I can't say that, I have to bite my tongue and ignore it.
I spend a fair amount of time doing my childbirth breathing (hey, there's a use for that longterm

) and repeating to myself, "Good relationship with the grandparents is more important than ____." Most of the time, it is. I have had to let go of some things - my kids will eat too much junk food while there, they will watch too much inappropriate TV and movies, and they will learn words I don't really want to hear out of their mouthes. But they LOVE their grandparents. They have a good relationship with them that has evolved as they've gotten older. And the values and beliefs that we've instilled at home stay with them, even with frequent grandparent interactions.