Due to prolonged use of the W-sitting, muscles can fail to develop properly. The consequences: inwardly turned feet and underdeveloped torso and back muscles. Children may also struggle to maintain their balance and may be unable to execute certain movements properly.
The issues and consequences can be treated by physiotherapy for children. Support soles can also help reduce the inwardly turned feet. The best thing is of course to prevent your children from assuming the W sit. This can be accomplished by having them use the cross-legged position or by having them keep both legs in front of them.