Ways to handicap in Magic the Gathering?
My friend and I have used the following and we think they work pretty well.


Start with a lower life total.  Start yourself off at 10 or 15.  As the kids get better, slowly increase it until you are back at 20.
Remind the kids about all their triggers or abilities they should be using.  Especially when learning, having someone remind you to activate a useful ability at the end of an opponent's turn or that a trigger with a "may" in it has happened is quite helpful in learning how to keep track of a lot of things at once.
Use simpler decks.  When teaching someone, introduce only a few concepts / interactions at once.  Starting with green fatty vs. white weenie lets someone learn the basics of a turn and how combat works without (usually) too many tricks via instants, etc.  Once they have a pretty good grasp on that, introduce something else.  Work your way up to the more complex things so as not to overwhelm them.
Stack their deck a little.  We will occasionally "fix" a child's deck when they aren't getting lands or creatures or something.  We don't mind because we aren't looking to win, we are looking to teach and practice.
Don't play as aggressively as you normally do.  When I play against my friend, it is no holds barred, all out play.  Against his kids, I might "forget" to use an ability or play a spell that would save a creature.  Not always, because they do need to learn how to deal with those things, just not every time.
Give the child a stronger deck.  Fighting an uphill battle gives you a little bit of a challenge.  You need to use your experience to overcome the power difference.