Guest List Fiasco! How to I resolve this nicely?

It is your wedding. If you don't want children there, then don't invite children. If you don't want to invite cousins past 1st cousins, then don't invite them. Just tell people that your venue can only hold so many people if they are rude and ask why so-and-so can't come. My 1st cousin was recently remarried in a small ceremony and didn't even invite 1st cousins. I didn't get up in arms about it as it is her wedding and she can invite who she wants.