Part of the problem with having the children that low is that you do a lot of bending over to serve and help them eat and that is hard on the back which is why getting them up to adult level with highchairs was the norm.

For our special needs child we used the harnesses with the rein - used those for everything to stabilize her. Harness goes on child and rein goes through chair wound tightly. Prevents child from standing up and although they can still slide forward if they are tight enough that will be limited and the table in the way will help.

If space is an issue there are children's tables that fold like a cardtable you could use and then stack the chairs. The advantage of the harness is that it prevents the total slide which just a seat belt doesn't so is safer for the youngest.

You would do better to just use an adult chair and the booster with tray on that chair and have the littlest just sit near the group that is at the childsize table. There are some boosters that fold up for storage.