If a father died and left everything to his children but not to his spouse, is that even legal?

A father is not required to leave any portion of his marital estate to his spouse.  The emphasis here is his portion of the estate. A follow-up emphasis is for portions of the estate not automatically disposed of by other means. 

Example, if father owns a motorcycle that he owned before he married wife, then that would be considered separate property and he could leave all of the ownership interst to his children.  However, if father and spouse also purchased a house together and the purchase agreements and deeds gave wife the right of survivorship on this death-- then wife could obtain ownership of the entire house because of that document... because rights of survivorship are automatic. 

I father and wife owned a RV that did not inlcude language of right of survivorship-- then father could only give his 1/2 interest in the RV to his children.  He could not give, even by will, her 1/2 interest.

The bottom line, what father can give and not give depends on what his ownership interest in the property is.  It also depends on what wife's ownership interest is.  To know for certain what property should be disposed of by will versus some other means (like right of survivorship), the children and spouse should have a probate attorney review all of the documents associated with the purchase and management of each item of disputed property.