[That's a great way to practice. How about sagging wrist (rest on the edge of the keyboard), any tip to correct this problem? Right now I hold a drumstick against the edge of the keyboard (parallel) and level it a bit higher than the base of the keyboard (where he could rest his wrist) so he couldn't sag his wrists.

Same thing. Pull up on the marionette string. The wrist should be parallel with the arm, not raised. Be careful, though. Trying to concentrate on too many things at the same time is more frustrating than productive.

Is that line called "slur"?

A slur is the curved line showing notes that should be played legato.

But what if there is no slur line or just staccato, should one look for a rest sign or a bar line or a long note as the end of phrase now? Is there a rule to teach kid how to see the music phrase or as they get older and have more experience, they will just know like an instinct. [/quote]

In most Western music, phrases fall logically in measure groups of 2, 4, 8 or 16. Four and 8 are the most common. There is often a pattern to the phrasing (2+2+4 or 4+4+8). Just because notes are staccato doesn't mean they aren't thought of as a phrase. You can often listen for the harmony (or the implied harmony) to tell where the phrase ends.