At what age does a child get a seat for themselves? What to do w toddlers?

What are you doing with your children 5 and under as far as seating. I have quite a few toddlers (3 and 4) taking up chairs. Should I put them on their mothers lap? At what age should a child get a chair at a table? I dont think my venue has the wooden toddler highchairs that you see in resturants and Im not willing to rent them. I figure by 6yr they are in kindergarten and could get a seat for themselves. Im still up in the air about anyone younger than that. Any help would be great.