Semi formal

Are jeans, dress shirt, tie and vest okay for a semi formal wedding. It's his cousins wedding, he's not in the party or anything. He planned on wearing nice dark blue jeans(literally the same outfit he wore for our wedding). Then we learned that isn't exactly semi formal. Is that still fine? If not for him, is it fine for a 7 and 8 year old, because even if we had dress pants, they aren't going to wear them. I've tried.