Just carry the smaller son in your arms when you go in then put him in the photo.
Don’t even acknowledge he’s there for the photo and if they ask just say the older one is having the photograph.
If it’s in the club shop, surely anyone can go in but not anyone can have the photo?
Then as your turn comes, just let the 2 join the photo.
This is one of those situations where being brazen will win. Go for it