How much force does it take (my 20 month old kid) to move a 165 lb adult in a office chair with casters (say 1ft) across a tiled floor?

Sounds suspiciously like homework, and last minute homework too with it being Sunday, so what do you think and how would you go about it.

Site rules require you to show us what you have and how you went about it.

Clearly NOT a homework problem.

The answer depends on the amount of friction in the casters. In an ideal world where you might be able to ignore friction it takes an infinitessimally small force to get the chair moving, as long as you don’t care how slowly the chair accelerates. Now, if you want to specify that the child gets the chair moving at some known velocity, say 1 m/s, in either a specified amount of time (5 seconds?) or distance (2 meters?) then we can help give a more precise answer.

Very little force. The toddler even has the height advantage, just the right angle. Good practice for the next several years, as the kid shows unimaginable powers. All very normal.