@spookymooky1 OP can you confirm whether my understanding is correct?
Your stepchildren spend every Christmas morning with their mother and then your husband picks them up at 1pm. They have already had an early Christmas lunch with their mother.
Your husband is gone for an hour an a half over lunchtime. This means you are alone at home with your child over lunchtime, not having Christmas lunch but presumably just eating something light to tide you over until later.
Your parents have their Christmas meal at lunchtime but you can never join in with this because it clashes with the time your husband has to collect his children from their mother's house, and he won't accept you going to their house for Christmas lunch without him.
Your step children then join you from early afternoon onwards. They have already had their main meal that day but you have not.
Someone, either you or your husband, but I suspect you, then prepares what is the main Christmas meal for you, your husband and your joint child, to be eaten later in the day, which your stepchildren only pick at because they have already had their main meal and are not hungry.
If you were to visit your parents in the evening you would not get a Christmas meal because they have already eaten their Christmas meal at lunchtime.
And your own child is young, so going round to your parents' house after you have eaten your Christmas meal in the evening is going to be after your child's bedtime and so not really feasible even if your stepchildren actually wanted to go and visit your parents, which they probably don't.
This could be resolved by alternating Christmases, or your stepchildren coming to you later in the afternoon, after you have already had your Christmas lunch, or by tweaking some aspect of the current arrangements in some other way.
Is that about the size of it?