Is it typical for a child to not be able to differentiate between a headache and nausea?

For young children, sometimes it is. Headaches can cause bad nausea, (I got headaches and would eventually puke). Young children are sometimes afraid to tell an adult about their headache, and then their stomach will turn, causing them to claim their feel "yucky". Since children are obviously not adults and had little experience in this area all they know is they don't feel well. They may complain of a tummy ache from too much junk food, but seldom respond to the Word 'headache." If they had been hit in the heat for instance by a Baseball then they would tell you their head hurts.