Hi, from what you told us it is not clear what the regression model is, but I assume you want to show that there is a significant difference in the slopes of the regression lines for the two age groups adult and child.

You can analyze all the data together and add a dummy variable for Age ( e.g 1 if Adult 0 if child) . The difference in slopes translates to a significant interaction between this Age variable and whatever X you might have.

I hope this helps a hit.