I have been with my girlfriend for almost four years and we have to kids together and we argue a lot so we decided to take a little break but still be together but now she is lying to me about little stuff... And I don't know if I should keep trusting her... What should I do?

Start by asking why is she lying.

Confront her. Before you do make sure she is actually lying and don't accuse.

How do you know she is lying?
What is she lying about?
What reason does she have to lie?
There questions will help to give you a thorough answer.