Mary ds have a designated area for children, not the best environment as you will probably know but at least u can get a pint and it's only acroos the rd from the ground, used to always take my young in' in there on match day n never did any harm.
Failing that why not try City square if the weather isn't too bad? Always stuff going on arnd there aswell, whatever you choose to do hope your little one enjoy his day and his first game, merry xmas!