After David showed first sign of interest in his sisters' highchairs, his urge to get on them becomes stronger and stronger over time as his mobility improves. Now whenever we sit down for a meal, David would get down of his own chair, climb up from behind Alyssa or Vivian, and try to squeeze himself in.
Since it will take quite some time to make one highchair for him, I came up with the idea of promoting all kids one rank higher last week. So now David has Vivian's chair, Vivian has Alyssa's chair, and Alyssa get to have a "grown-up" dining chair. Everyone is pretty happy with this arrangement.
David is getting so good at climbing up and down the highchair I made that at mealtime he'd go to the dishwasher, pick out a plate for himself, and climb up the highchair while holding on the plate.