I would begin advertising towards the end of this school year. I do not recommend beginning with discounts. Why would your childcare be worth any less just because you are new?
Go now as you intend to go forward.
I don't charge an enrollment fee as everything is included in tuition. Most that charge those use them with the expectation of parents using that to cover their last two weeks of childcare. I prefer the pay as you go format, payment up front on Mondays in advance for the week. If they leave mid-week, that is on them. No refunds are ever requested or due.