Children are built-in cute machines. If they are around and involved in anyway at your wedding cuteness could break out at any point and spread through the pews or chairs and infect all of your family and friends with smiles.
Kids are a great part of wedding photos. Look at your own family’s wedding photos. You might even find a picture of your parent as a flower girl or flower boy. Cute!
If they are the bride and groom’s children there are deeper emotional, psychological, and etc. etc. reasons to have them in the wedding. But also… cute! I’ve been to weddings where the bride, bridesmaid, or groom held their fussy child during the entire ceremony. Like I said, cute!
Children talk loudly and say things that make us laugh and think.
Children have relatively low dexterity which makes flower tossing, ring bearing, and sometimes even standing in suits and dresses super cute!
Some children take the occasion very seriously, so then when they trip, wipe their nose, or generally do something silly (because children are naturally silly) the contrast makes all of the adults remember… oh yeah, we’re a bunch of silly people inside and we’re here celebrating the silly and ridiculous love of those serious but silly people up there. Cute!
So, unless the child in question is an incredible terror there are more reasons to have them in the wedding than not.