OP - apologies for taking ThePersuitofhappiness post at 22:52: at face value and not checking the facts - ie that you had not in fcat checked it out. However it still stands that:
YABU - for CHOOSING to AVOID clarifying the invite status of DS1 with the bridal couple however uncomfortable this would be for you. If you assumed it was a typo you could have sent a simple text at any point to clarify. This not a good enough excuse and you know it.
YABVU - for then PLOUGHING ON and taking him despite choosing not to confirm arrangements. Effectively GATECRASHING a wedding.
YABVU - to then COMPLAIN that there was no place for DS1 when YOU CHOSE not to clarify. You should be APOLOGIZING to the hosts for YOUR misinterpretation/misunderstanding.
YABVU - for not assuming "POSITIVE INTENT" re the family invitation - ie that the couple kindly asked youngest children to make childcare arrangements for easier guests. And that they have every right as hosts and payers to invite who exactly they want - and you should respect this.
YABVU - to complain about helping with their toddler - either volunteer and DO IT GRACIOUSLY or say no if you dont want to do it. Did you do it to assuage your guilt for using your DS1 to stubbornly gatecrash their wedding?
YABVU - to complain about the seating plan - guests are expected to MAKE AN EFFORT to mingle and socialize with new people to make a social event a success in response to the efforts of the hosts.
YANBU - AT ALL to expect your toddler to be fed at an event.
You are CROSS because of the outcome of actions you chose not to take (checking invite status of DS1) and then actions you chose to take (taking DS1 regardless, looking after couples toddler)
Looks like a passive/aggressive and in-assertive approach which is a recipe for naval gazing, blame and inner bitterness.
Looks also like someone who takes more than they give and have unrealistic and shewed expectations of others.
You will be the talk of the wedding among your mutual friends and their family as being an exceptionally ignorant and stubborn guest who blighted the efforts of the hosts on their wedding day APPALLING. Clearly the hosts have much more sophisticated social decorum that you by their efforts to make light of the difficult situation you put them in.
How would you view a guest who did this to you on the most important day of you life (one which involves difficult financial and social decisions and logistics) and them went on to complain about it?
Would you think that they were BU?