system
A chat on subreddit /r/NoStupidQuestions
human
Asking rent of your own kids is just such a stupid move; most of them expect you to "Return" what they spent on you in your toddler phase, when they go into Retirement.

Most of them just do it, because it "Supports you in creating your own life" but that’s just BS. In the current economic it’s just no longer feasible to live as a single person (less so with a baby) in a separate apartment.

They are completely out of touch. It’s not 1960 anymore where a singular working person pays half its income in rent and the rest is to throw out and window and snort high grade cocaine with.

As long as you are pulling your weight around the house, and offset or pay for the food and partially pay for water/Electricity, you and the child are consuming it’s fine.
It’s not like they make any loss because of you moving back in; what were they gonna do? Subrent to someone?

I‘d be fine to agree (non contractually) to buy groceries (in moderation) with your income so that they are not "Feeding" you, but paying rent is just borderline ridiculous…

Edit:

Alternatively explain to them that you were hoping to save some money for your kids college, and try to guilt trip them (but then actually put it in a college fund).
gpt
I tend to agree with this. The concept of “you turned 18, you’re on your own!” is wild to me assuming the parents don’t have major money issues. 

I had a couple friends who “rented” from parents in their 20s but their parents just had them dump it into a savings account. It was returned to the kids when they needed a down payment. It seems like a good compromise because it keeps the child in the mindframe of budgeting for housing, is “fair” to the siblings, and forces savings at a time where most people burn extra cash on booze and ski trips.