He should have made it clear from the beginning, that was wrong but other than that, there is nothing wrong or unusual in not inviting your children. They are well above the age of exclusively breastfeeding!
Do you or your wife never go out or socialise without your children? Do you take the school holidays off work to avoid leaving them with other people? If the answer to either of these questions is “no” then ask yourself why you can be arsed to make alternative arrangements for your children on these occasions but not on the occasion of the wedding of your best friend of 40 years.
How many wedding guests do you reckon have children? It’s probably quite high, if they were all invited, your friends would be spending $120 + per head for a lot of bored children, many of whom they have no relationship with, to attend their wedding, while cutting real friends and family due to budgetary constraints.
Children always think they will enjoy adult parties and weddings but the reality is lots of sitting around being quiet (hopefully) during a ceremony, then lots of standing and sitting around while adults make boring speeches and talk to each other.
Also your children may know how to behave thenselves but I bet your friend knows family or friends’ children who can be guaranteed to race around during the ceremony, lick the icing off th cake, dominate the dance floor and strop when they are bored, all while their parents carry on drinking and chatting/ occasionally say “Riley, stop that!”/ look on proudly at their little characters stealing the show. How do the couple discriminate?
Lots of children at a wedding often means lots of guests leaving early to get them to bed, and conscientious parents not really letting their hair down and enjoying themselves because they rightly have to see to their children.
I think your best friend and his fiancee have chosen to host their dear friends and family in the way they believe they will most enjoy themselves. You can’t please everybody. Either you value him enough to go along with it gracefully: their wedding; their rules or you lose the friendship.
You and your wife presumably had the wedding you wanted, perhaps with children at the heart of it, but this is their day. One day. Or if you are travelling then a couple of days.