I was never given an enema as a child, even at the hospital, but I love the scenario of a mother or an older sister giving me a severe punishment enema, usually with a hairbrush paddling while holding it.
My favorite punishment is high, hot, and a hell of a lot with mild bar soap dissolved in it, though when I say a hell of a lot, my capacity is not what some peoples is. I don't think I could hold two quarts if I tried.
Kneeling in the bathtub or OTK with the punisher sitting on the toilet lid are my go to positions, but I also like, for more severe enemas (more soapy, hotter, et cetera), to be made to stand with my legs spread, leaning forward with my hands on the wall, while the enema giver lubes me slightly and carelessly jabs the nozzle in, then stand there moaning while it fills me up.
It should go without saying that any significant squirting is a further paddling, and failure to hold is another enema, for as many times as it takes, with stern words and shouting.
Another version is a school punishment, where I pretend to be girl in a strict private high school and whoever I'm playing with is first the teacher, then the school nurse, and they first humiliate me "in front of class" by viciously berating me and asking "other students" to shame me (did I mention I like to have people watch me when I'm in a BDSM scene?) and then send me to the nurse with a note that says "severe constipation," which is actually code for "little Katie needs her ass punished with a lot of hot water."
I admit to adoring the very skeevy video (I think it's Czech) called "Detention House" where a bunch of schoolgirls in a victorian school get lined up and enema'd in a standing position for punishment, then caned severely in bondage, before getting their caning wounds rubbed down with a stinging ointment. It's humiliating, would be severe abuse IRL, and looks extremely painful, but how I would love a dom or domme with the resources to treat me like that.
I have recently gotten into a little humiliation diaper play (I try not to actually mess them, because that's a turnoff--sometimes I will wet them on purpose) and very often I'll get diapered after my enema to prevent leaks.