This is definitely not every child’s (or adult’s) favorite part of the lesson but it is a fantastic way to get used to the guitar. If playing guitar is ice hockey then the crabwalk is the equivalent of skating laps.
The crabwalk builds strength, dexterity, coordination and serves as a warm-up: even to this day I get ready to play sets by doing crabwalk for 3-5 minutes. With small children I get them to start with one finger to get them used to pushing down frets, then we’ll move on to different fingers (one for each fret.) I also like to try timing it (4x) and letting students beat their own records.
There are many variations when a student has mastered it, try backwards as well, skipping strings, holding the fingers down the entire time and more.