I know it may not be possible to store away everything before showings, especially when you have little children. However, it is very important that your house is as organized and inviting as possible. When people are house hunting they're imagining a fresh start and that becomes difficult with toys scattered about.
Also, don't forget that some families might not have any children and could be turned off by unsightly toys around the house. So be sure to pick up and put away those toys before potential buyers show up.