I wonder if this is making you feel that your child isn't being accepted into the family in some way? Whether it feels almost like a rejection of them or your new family unit?
I can understand that, particularly when your family is still so 'new'.
If that's the case, you need to separate those feelings from this situation because I think it is now pretty common to have a completely child free wedding. I don't understand that view personally, but it does seem to be growing in popularity and I think if a couple prefer not to have children there then it doesn't matter whether they are family or not.
So, you need to accept, it's not your wedding and therefore not your choice over whether your child attends.
Save your battles with in laws for things you may be able to change - maybe further down the line when you may need them to get their heads around why you need to parent differently (and they need to Aunt/Uncle/Grandparent differently).
My children, at any age, would have struggled with me leaving them for a whole day - but June is a long way off still and your situation may change.
My children also wouldn't have coped with the behaviour expectations, unknown people, number of people, photos, speeches etc at a wedding
Perhaps, right now, you could make the decision that your husband will definitely attend the entire event. That is a clear and safe (as in highly unlikely to have to change) decision to make and may settle your mind a bit and give clarity to the bride and groom for initial numbers.
Maybe you could also state that, by June, you may also be able to attend either just the ceremony or the evening celebration. By June, your child might be able to manage being looked after for a few hours by someone on your side of the family - and you may welcome a couple of child free hours out with your husband.
Whatever happens by June, I would really caution against adding extra stress and strain on wider family relationships for the sake of one day that in 2 years time you will barely remember (but they will forever hold a grudge over).
You will have many amazing family events over the years which you will be able to fully enjoy as a complete family unit, whether just the three of you or with extended family and friends.