26" for smaller 10-11 year olds (most people skip over this and go 27")
27" for adults or bigger 11-year-olds
Yellow/Red: 6-year olds (aka Stage 3 or QST 36 (shoot for the non-foam version)
Yellow/Orange: 8-year olds (QST60)
Yellow/Green: 10-year olds (QST78)
Standard balls: 11+
For balls, the numbers 36, 60, 78 refer to the size of court typically played on. The 36' court could probably be in your driveway! The 78' court is a full-sized tennis court.
I've searched YouTube for great videos, but I was shocked to see that there isn't that much content available. Feel free to search, you may have better luck than I did. Most of us here at Sherwood Tennis focus on high-school kids, so the littles aren't our specialty either (Our resident tiny-tennis expert, "Bubblehead" may be able to put something together soon, but I'm not sure). Another option might be your best bet.
WebTennis24 has a section dedicated to coaching kids, including a series called My Daddy My Coach. Some clips I've seen look like he's got good ideas. Check it out here. You can join for $10/month and there is a 14-day money back guarantee. For what most of you are looking for, a single month of access will probably get you as far as you'll want/need. If you decide that you want to keep going, great!
With the Covid-19 restrictions, tennis in Sherwood is booming. A sport that is played nearly 80-feet from another person is great for social distancing, but you'll still want to be careful about touching your face since you and your opponent are both using the same balls (and a thorough hand-washing after every outing is encouraged). Restrictions seem like they are about to get tighter, where even interactions like tennis may only be allowed for people who live together. So, families looking for new activities may want to consider tennis.
10 years ago, teaching tennis to kids was pretty difficult. Small rackets existed, but the balls always moved to fast and bounced too high. Now, they make balls specially designed for kids, and it is a total game-changer. They are bigger, slower and have limited bounce, which lets kids learn to take full swings and learn to keep the ball in play much longer than you'd expect. So, between the isolation/search for family activities and tennis becoming very approachable, there has never been a better time to teach your kids to play.
First of all, supporting small businesses right now matters to them so much. A local racket shop (Players) is going to try to put some packages together over the next few days. Please consider taking advantage of that. He'll be able to put together packages that meet a variety of needs:
Junior rackets (for different ages)
Adults rackets (varying between top-of-the-line and "I just want something cheap to hit a few balls with my kid")
Balls for specific ages
If you are interested, contact and I can get in touch with him even if his shop is closed.
If you really want to shop online, tenniswarehouse, midwestsports, tennisexpress are the typical tennis-specialty sites.