I'd speak to the bride herself. & I personally don't think I'd go if they're going to be like that.
When DD1 was 5 months old I took her too my uncles wedding & reception even though children weren't invited.
His poor wife got a shock at my wedding, lol...bring the kids! The more the merrier! Let them all be feral together!!
To say we'd prefer no kids is one thing, but 'not welcome'? They should be taking into account the age of your DS & the fact that you have to travel so far. He's not exactly a toddler whose going to be running around sticking his fingers in the cake & skolling everyones drinks!!