I would do one of two things based on which is more comfortable for you:
1) Ask for a table right next to the buffet. That way you can run up to grab an extra bagel or 'quickie' thing while child is only 20 feet away and in eyesight.
OR
2) Just load up your plates high the first time up there....so that you either don't have to return OR only have to return one time. Then just take child out of high chair and bring him/her with you back up. Of course, the child would have to walk on his/her own to give you 2 hands to hold the plates. But a 2 year old should be able to handle that.
I guess if you need or want to carry him - just hold him one-armed and load up one plate...bring back to table then go back up and load up another plate and bring that back.
It'll work out - just might be a little more cumbersome than if another adult is with you.