There could be several reasons why your ex does not want your child to
play with guns. We do not have all the information to decide if she is
right or wrong. For example, if she is living in a dangerous community
with lots of gun violence then I could see her opposition to her six
year old playing with a gun. She could also be a victim of gun violence
or is very close to someone who has been a victim. Untill we understand
why she is oppose to the idea we cannot say she is right or wrong. I
would suggest talking to her get her concerns and try to discuss your
point of view and go from there.
Good luck