Once in a filmmaker's lifetime, he or she will most definitely work with a kid. If you know what you're doing, you'll have an amazing performance.
However, many things can go wrong when working with a kid. This past week, I had the experience of directing a Christmas commercial for work. Our concept included working with a 5-yr old, which pushes the edge of whether a kid can successfully take direction or not. Thankfully, it was only a 30-second spot. For this spot, production took 4 hours.
Kids fuel on incentives. Find out what the kid likes, and come prepared to the shoot with things to keep the child engaged. Be careful on using this too much, because the kid will start to get over it. Give subtle reminders for them to work towards a goal. For example, food, gifts, or a quick 10 minute break. Also make sure the parents are OK with this.
In one shot, we had to get the kid running down the stairs towards the Christmas tree. Instead of telling the kid exactly what to do, make sure you get him involved and make him feel like he is part of the team. In my case, I asked the kid if he knew how to count to 4. Then, I made him go onto the fourth step of the stairs, counting as he went. Once there, I gave him some direction, and asked him to count down from 3 and say "ACTION".
This worked for us because he felt like he was in control, and that he was helping direct the commercial.
With any kid, you are not going to be able to get a lot of takes. Repetition is not something kids enjoy, and generally speaking, after around 5 takes the kid will be tired of it. You need to plan your shot ahead, and practice your movements, focus, etc. After 3 or 4 takes, the performance of the kid will suffer.
LET THE KID BE A FILMMAKER TOO
Kids LOVE cameras. Try to relate to them and ask them about their favorite movies or TV shows. Flip the monitor around once in a while so the kid can see themselves on the screen. Or... maybe let them clap the slate or yell "LIGHTS, CAMERA, ACTION". You never know what will excite a child.
I hope you found this helpful. Enjoy these screenshots of the commercial.