Please please please don't put the highchair ins the corner of the room. If you can, the tables nearest the doors are a fab idea.
I just went to a wonderful wedding yesterday with my 2 yr old and 4 mth od baby. The baby was freaked out by a the noise so i had to take him out frequenttly but was trapped in the corner. Everytime I had to go out, 3 people had to let me out, so I ended up missing a lot of the food just because I didn't want to keep bothering them. Surprisingly my 2 year old was fine but every other child needed to get up and stretch their legs away from the table regularly too.
Your guests with young children will thank you (or at not curse you)