I’m conflicted about inviting one of my guests to my wedding

Well really the question is do you want a child at your wedding? Are there other people bringing kids? Have you been asked and said No to other people wanting to bring kids? Is she from out of town and wouldn't know any babysitters in the area? If you do decide to invite them, you'll need to have them welcome to all of it, to leave them out of something would be rude.

If you two decide to do a child-free wedding, then this guest can decline attending. But, it has to be all or nothing. If you're concerned that this particular child is too young for the ceremony and may cause disruption, most considerate parents will remove themselves in the event that they can't be still. But, you'll be so into getting married to the love of your life that these potential anxieties will be minor in the longrun. Best wishes.

It sounds as if you are committed to including the cousin's son. If so, the invitation would be addressed to the parents and the son by name. If he's invited to the wedding, then he'd be included for the duration and it would be their responsibility to supervise. Do you have reason to believe he will be disruptive? How old is he? One would hope any considerate parent would remove a crying child from a wedding ceremony.

The other question is whether you'd be offending other cousins or siblings with children if this boy is an exception. Are there any other children in the extended family? If so, I would reconsider inviting him. Either the parents get a sitter or they don't come. Another option is to hire someone onsite to watch young children if they would be open to that compromise. Or the parents could take turns watching him.