Tennis isn't the easiest sport for young children to learn, but if they start right, they're more likely to continue playing and enjoy the game.
The main goal is to make sure they're having fun and the best way is to let them experience success.
When practicing their ground strokes, shortening their backswing can help them connect with the ball and ensure a normal follow-through. As the child improves, gradually lengthen the backswing to a full forehand or backhand swing.
You can start with a drop feed (either you or the child can do this) then progress to a short toss feed (throwing to the child) then eventually feed from your racquet.
The next step could be feeding from across the net with the child well inside the service line; as they improve you can move them back further. After a few sessions get the child to hit the ball on the move; first with a couple of steps and then on the run. Some kids actually hit better on the run.
If the child is not finding it easy, end the hit with something very simple, such as volleys.
With accurate feeds, even the least co-ordinated child will get volleys to go in. Keep your hits brief. Half an hour is often enough for ages 4-6.
Make tennis fun! Children sense if you don't think they're doing well, this can discourage them from taking up a racquet ever again.
All comments should have an element of praise.
If you need to make corrections on shots, start with something positive then say how they could improve it more.
Children enjoy achieving easy goals, like getting five forehands in, or even just connecting five times. Whether it's with a group or individual this is fun for them.
Always play games after their hard work, choose 'no lose' games to start with.
As your child improves you can introduce them to more competitive games.