E.B.
I'm sorry your mom is so ill.
But if your child is young, your child will most likely not have a true concept of the pain of impending loss. Even if your in-laws and husband were to describe to your child the sorrow, the physical problems, chemotherapy, the looking back over a lifetime of memories, your child would probably not really understand. (If your child is a teen, that's a different story.)
But right now, your child and his or her dad and grandparents are building the kind of memories that you have with your mom at this time in her life. They're keeping things normal for your child. They're showing love. If they were to sit and weep and stay home out of respect, it wouldn't make sense. They probably are a little at a loss about what to do, so going out seems to make sense. Keep busy, keep active, keep the child happy, tire him or her out, and keep the household going.
Yes, I'd be angry. Angry at the loss I was inevitably facing, angry that a loved one is suffering. But not angry that my child was happy and that people were showing him or her joy and family, and making memories. I'd be thankful that my child had family to love and be loved by.
After my dad passed away, I remember going to a supermarket for basics for my kids. In the cereal aisle was a young family who were laughing and having a very nice time. It was all I could do to scream at them "how dare you enjoy shopping and laughing at the funny cartoon bird on the cereal package! I've lost my father!" I realized that my pain was not their pain, thankfully, before I lost it in the cereal aisle. And you're in a different town now, with your mom, not near your family, and that's hard. Try to focus on her, and be grateful.
I bet that your in-laws and husband are determined to keep life going as best as they can, and I bet that they are very aware of the situation your mom is in. But I also think that they know that to sit and mourn now would confuse and hurt your child. I suspect that if the tables were turned, you might cook and clean and take your child to a movie and try to keep things as normal and pleasant as possible, so that your husband could concentrate on his ailing parent.
Again, I'm sorry your mom is facing this battle. I hope she is comfortable and is able to be comforted by your presence with her at this time.