There is no legal right to be a member of the forces, or to engage in combat. And differential treatment based on having dependent minors is already part of numerous laws and government programs where justified (think welfare, social services, tax policy, immigration laws, etc.)
I appreciate your sentiments re: WWII. In that war, men with children were not conscripted, or conscripted last, though (I think) they could volunteer.
As for the terrible effect that having a parent go to war has on children, here's a report from last year:
A mental health crisis has erupted at the Petawawa military base, where children are "on the brink of suicide," Ontario's ombudsman said Friday as he released a damning report on the state of Canada's military children.http://www.cbc.ca/canada/story/2007/04/13/children-ombudsman.html?ref=rss