Is it permissible to allow children to play/build with legos on Shabbos? Is it permissible to allow children to play with a Rubix cube on Shabbos?
Yes, both of these games are permissible on Shabbos. With lego, if the child is old enough to understand he should be instructed not to create a structure that contains an “ohel tefach” a roof with a space of about 10cm underneath.
In addition, lego above refers to standard playing where the norm is to build and take apart often. One who builds a model plane or other that is meant to be kept intact on a permanent basis, can not be made on Shabbos.