first: not every thing is solid,liquid or gas.In the universe exists a 4th state plasma which is most common.
Electricity is a kind of interaction between particles with spin 0.5(particles that form matter).The interaction of particles is basic a change of particles with spin 1,2,3,⅔,3/2 etc.(this particles are ’‘virtual’’ ,they can be observed only in special cases).This is the scientific explication given by Stephan Hawking,but to your 7 years old daughter you can say that electricity a way for particles to communicate.