I think that it is rude to not have included your DH, at least for the wedding ceremony. If there was an issue with limiting numbers for the receptiona t least an invite to the wedding should have been given.

I agree with Tracey. Just tell them that you are not comfortable coming by yourself and leaving Lachlan at home with DH and if you bring Lachlan you would need DH to help you with him so that you could enjoy the wedding. This is a day to celebrate the union of two people and you wish to share that with him, not alone as you are no longer a single woman.

I don't think it would be rude of you to ask if he could attend the ceremony, go together as a family and not attend the reception. If you get a knock back on the ceremony, send a decline and a token gift to show that you want to celebrate your friend's wedding.