I was usually given a choice of something like an enema, or a high dose of Milk Of Magnesia, or choice of an enema or a bisacodyl Suppository or something similar.
When I was not given the choice I was allowed to choose between two times.
Example: I got home from school and was told I could not eat the rest of the night and we could do it at bedtime, or I could hasve a lighter than usual dinner and we could do it in the morning.
My dad rasied me as a single parent. When I did complain, he would say, "You don't honestly think I LIKE doing this any more than you do?" or, "I don't want to do it any more than you do, but it's what you need, so lets just get it over with!"
Also, I was always encouraged to watch, and by the time I was 8 participate in the mixing of the enema. By then I was also carrying it from the bedroom to the bathroom, and attaching the nozzle.
It was made clear, "the sooner you get in position and get relaxed, the sooner it will be over."
When was nine, and I was expecting the enema to be just like the last, just as I got into position, he handed me the hose with the nozzle I had put on a few minutes earlier, being careful not to touch the nozzle itself, and said, "You DO know it has to go all the way in, and point towards your navel, don't you?" said, "Yes" he said " You want me to turn by back while you are getting it there?" I said "That's OK." and just lay there a little surprised, He finally said, "The sooner you get it done the sooner it will all be over!"
That was generally his approach when I resisted. It was like he was getting me to do it myself, so I would not have time to think of a way to complain too much.