As a parent of a daycare aged child, here are my thoughts:
-Depending on the types of daycares you're considering you can try to get in there a couple times a year. Kids change so quickly when they're young that every six months (compared with every year for school aged kids) is not unrealistic.
-Depending on how much effort you feel it's worth to you (and to your sales), bring a couple backgrounds with you. For one picture day we dressed our daughter in a cute mostly-brown top. The photographer that day used a tan backdrop - the images looked really blah (and that's being nice - even though the photographer captured a really cute smile there was no way I was buying and displaying that picture). You may feel it's worth your time to swap backgrounds once per group if needed so you get every kid standing out well, or you may decide that you're more willing to write off sales to the few kids who may not work against your backdrop of the day.
-Just like with older school pictures, what I've seen are print packets (8x10 sheets) sent home, usually with 2-3 different images over a set of different sizes (wallets, 4x6 or 5x7, and 8x10). The images are watermarked in the lower corner - you don't want to splash a heavy watermark across the center. Remember that you're selling to parents, you don't want to interfere with the view of the child (these aren't photographers who are able to look past a watermark to evaluate the image underneath). The parents can pay for part or all of this packet (and keep these prints) and/or order additional prints. If they choose not to buy any they just return the print packet to the school.
-You're right that catching the parents for a sales presentation will be next to impossible with the variety of drop off and pick up times necessitated by the parents' schedules. I also expect that you'll get next to nobody pre-ordering...who's going to commit to buy photos of kids (notoriously uncooperative) before they know that the photographer was able to capture the kid on a good day? If parents are committed to spending money on a professional photographer I'd expect them to schedule a personal sitting, when they can be there to be sure of the shots the photographer is getting.
-In the interest of full disclosure, I'll admit that I've never bought any of the packages our daycare has sent home, but that's because I'm more judgmental (and already have more quality pictures of my child) than other non-photographically-oriented parents.
-The one thing I do really like about picture day is that, included with our print package but free for us to keep regardless, is a wallet-sized identification card with a small portrait on it for the child. It has places to write in your child's stats (age, weight, etc), steps to follow if your child goes missing, and references to access the photos (say, for posting on an Amber Alert). It says it is "compliments of your child's school in partnership with: Smile Safe Kids, National Center for Missing & Exploited Children, and LifeTouch." I don't know who covers the costs for these but I would expect it's not the photographer - however, if you can present this as something you'd be willing to work with while pitching your presentation, it might be a useful selling point to get the facility's business.