Just Dance 2018 is set to feature a mode exclusively made for children and preteens. Nothing is revealed about the mode.
Here is the official info from the Ubisoft website:
"The youngest players will have a blast with the most fun dance playground: the brand new Kids mode for Nintendo Switch, Wii U, Xbox One and PS4. 8 new routines were created with the help of kids’ development experts to encourage healthy movement. Thanks to a tailored interface, scoring, and special characters, kids will be able to safely enjoy the game in complete autonomy with positive and supportive feedback."