@nikiboo druids don't polymorph. they use wildshape to turn into a beast. as it would happen, you can also only use polymorph to turn a creature into a beast. in order to change a creature into a different creature, you would need to use true polymorph, but you can't change age with true polymorph, just what kind of creature they are. the closest you could get to "polymorphing into a child" would be to use true polymorph to transform into a halfling.

perhaps there is some illusion spell that could give you the appearance of a child, but illusion spells only work well if not interacted with, because they are illusions. i could use disguise self to turn into what looks to be a 4'10" child, but it would be a purely visual thing and any interaction would result in the same feeling of an adult body, including any clothing. that's why when you use disguise self you typically only want to change the way your face looks or something.

a changeling gets somewhat close to being able to do this, but they are constrained in the way that they cannot change outside of their size class, which is medium. a child will typically be classified as small. so they can't go super young, but maybe like 12 years old or something like that.

there is only one way i know of to actually make yourself younger in age (that isn't homebrew) and that would be through the use of the wish spell. that seems incredibly dangerous and frivolous though, since you can have monkey's paw effects as well as the possibility of losing the ability to cast wish altogether. if you are a level 17 wizard or bard, you won't be casting wish to do things outside of its normal constraints unless you're desperate and realize you may never cast it again, giving up ultimate power for whatever reason you might need to. transforming into a kid to have sex with someone isn't worth it lol, especially because for the time you specified you wanted to be a kid you would be completely defenseless, and you don't make it to level 17 without making a few enemies. doing that would be incredibly foolish.

dragonirian isn't right, but he's got the right spirit.