I used to practice piano for hours on any given days. Only recently did I realize my time and efforts were not put into good use. Here are a few suggestions that might help you or your children to better make use of their practice time.

1. Only practice a portion at a time. Have you heard of the saying : "How do you eat an elephant?" - a bite at a time. This is true in any given task. Do not practice the whole piece in one day. Breakdown the piano piece you are going to learn in many portions. You will need time to digest any given measurements. The fingers need to familiar with the location and touch of the notes.
2. Analyze the piece - You may analyze the piece by observing any repeating segments. Very often a music piece has repeating segments. Sometimes the repeating segments might varies slightly. By doing this, you achieve the goal of practice a small segment but covering the repeating portions that appears later on in the same piece. Also analyze the forms of the piece. A common music form is ABA , AABA, AABB, ABCA etc.
3. Chord/Cadence analysis - The same chord may appears for a few measures. This is true in any musical period (Baroque, Classical, Romantic, Modern, etc). If you notice a same chord is repeating for many measures, then the notes within those measures belongs to the same chord and scale. This also helps your sight reading abilities. You get to predict the chord and notes ahead of time.
4. Warm up the fingers by playing the key of the piece - If the music piece is in the key of A major, you may want to warm up your A major scale by playing them numerous time to have a feel of A major's tone and fingering.
5. Write down the three most important chords of the key - If the piece is in the key of A major, you may write down the tonic, dominant, and subdominant chords respectively. They are A chord, D chord, and E chord. You will soon discover the left hand part of the music comprised mostly of these three chords. This is true in any given music. These three chords are
   mostly used and harmonized.
6. Practice each hand separately first - This method is most useful if you are playing pieces from the Baroque period. Music composed by Bach have many themes. Very often each hands is playing a different subject. Practice slowly in the beginning, speed up once you have a good understanding of the piece. This is important when you have a music piece that varies in rhythm (going from quarter note to half note, dotted quarter note, etc).
7. Do not lose your sanity - Some music can be frustrating to learn. Keep a positive attitude. Though you are practicing a small portion at a time, you are still making progress. Some advanced repertoire may take 4-6 months or even longer to master.

I hope the above suggestions help make your practice more productive. To learn more practical guides to piano playing to improve your piano playing