Place stickers with the letters A-G on the appropriate keys at the center of the piano or keyboard. This will help children sync the piano keys with the notes on the written sheet.
Start with the top of the staff (right hand). Encourage them to memorize the standard catchy phrases like "Every Good Boy Does Fine" for the lines of the top of the staff and "FACE" for the spaces (not forgeting to mention the middle C just below, of course). These phrases really do help kids remember what each line and space on the upper staff represents. Don't forget to mention the treble clef, too. You can show them how to draw it for fun. Allow at least a week or two of repeating the phrases daily for them to really get the hang of it.
Explain the basic time signature 4/4 (4 beats, each a 1/4 note long) as well as simpler rhythms such as quarter notes, half notes, whole notes, and the same time lengths of rests. Have them clap or snap their fingers to represent the time signature (as a sort of metronome), then have them clap/snap and sing each type of rhythm using the word "da" at the same time for practice. Then go a step further and have them clap (as the time signature) and sing the rhythm of a written song such a nursery rhyme.
Have them read and play songs that only use the right hand such as, again, nursery rhymes. Mark the letters under the notes as sort of a cheat sheet early on. As they get better at it, you can stop writing the letters underneath.
Introduce the lower part of the staff as well as the bass clef. Show them how to draw the bass clef for fun. This time, have them memorize the phrase "Good Boys Do Fine Always" to learn the lower lines and "ACE-G" to learn the spaces.
Have them play simple songs that involve both the right and left hand (both parts of the staff), although with more focus on the right hand. It'll take some time to get coordinated, but with plenty of practice time, it'll become easy as pie.