Even tho I think having children is the best thing in the world I still feel one should try and do his best to be stable in his life before accomodating kids in his life. Make her realise that what you both have right now might not be enough for what you'd like to provide your children with. Will you have enough money/conditions to be a familly together on your own house? Will you have enough time when kids start going to school and help them with school works, weekends out, do stuff together? (they need all of your time) What about college? What if something goes wrong with him and he needs more then the usual? Yes please think first about the children and then the relation, make her understand.
You should just tell her you dont want to yet. She will be fine lol. Say exactly what u say on here. If u dont tell her then thats gonna be a sticky situation you put yourself in.. wouldnt you rather her be a little upset then you be put in a situation u dont want yet for 18 years? .