If you're constantly losing to your kids at Halo, have no fear, there are ways to obtain victory and regain your self respect.

Steps

If you're smart, you won't play them at all. They're younger, have better reflexes and more time on their hands. (the young at heart keep reading)

You should practice on your own preferably when the kids are sleeping. Never let them see you practice; that will only make them practice more or tease you more when they beat you.

Play through the campaign mode in CO-OP with your kid. Not only will you get better, but you get to see what they do and live through it.

Never play one of them alone. If you don't have two kids, grab a neighborhood kid or reasonable facsimile. Preferably one with equal skills to your kid.

Before playing, establish an alliance with one of them. Or do your best to generate some sort of competitive animosity between the two kids. This is a very important step as you cannot win alone. Use any and all tactics at your disposal to accomplish this. Bribery is a perfectly acceptable means of accomplishing this goal.

Once the game begins, fuel the competition by commenting on every small victory that either of them have against one another.

Gather grenades and the best weapons.

Use the grenades and the weapons when their health is reasonably low so that you get easy kills. This method is known as "cherry picking".

Repeat steps 2 thru 7 until victorious

Tips

Good luck

A good tip is to get someone else who is older and go against your kids in team mode. Have the person on your team never leave your side. This forces a 1 on 2 situation whenever you run into one of them.
Don't make it obvious or they will try use the same technique. When you beat them just remind them that discipline and tactics will beat out youth and energy every time!

If you're used to playing games on computer, this may help.

Remeber your kid probably has less patience than you do. Use a tactic known as 'camping'. Hide somewhere with a sniper weapon, and a powerful close-quarters weapon - try and get them with the sniper before they come into range of their own guns, then go hide somewhere else. Never let them figure out quite exactly where you are hiding.