Aleysa,
I am an adult adoptee and also a birthmother. I'll try to give you my take on things
I was always told I was adopted, around age 7 my amother told me the 'story" about my bparents, I later found out that what she told me was untrue and that was very difficult for me to find out.
I reunted with my bfamily when I was 28. There was vital medical information that i needed to have and had my parents kept my adoption a secret, i would not have found out this information and it could have cost me my life.
I wouldn't tell a child the gory details, but I would give them the information over time and in stages as they grew. Keep the information age appropriate. This adoptee wanted to know all the details whether they were good or not.
There is a saying over here "You are only as sick as the secrets you keep". Living in fear of the child one day "discovering" they are adopted, or maybe a medical issue comes up and you have to say something, living in that fear can weigh down your spirit and ultimately the child will pick up on it.
Good luck on your adoption journey