You can't make them - I'm lucky, in that my 3-year-old loves flying and can't get enough. Recently he made his first trip with just me, so he sat in the front, and was as good as gold.
I have, though, following that trip, made a vow not to attempt single-pilot IFR with just him - trying to monitor two radio frequencies simultaneously*, plus his constant questions and 'look, daddy, there's the top of that cloud. Whee!!!!' (as we bump through it) was just a little more than I could manage comfortably! (Also, he loves the sound of his voice through the intercom and sings himself songs as we go along...)
*serves me right - when the nice lady from Brize said 'are you able to contact Gloucester on box 2, they appear to have instrument traffic in their overhead' I should have said no, but I wanted to show off that I had a fancy audio panel...
Tim