If the first child is intelligent and a competent designer, then the second child will be safe. There are no issues here. Just 2 children who trust each other.
My sister told me I could fly after watching Peter Pan. I could not fly.
Skill issue
Sounds like you didn't believe you could fly.
If … that word carries a lot of weight with random children’s ideas
"If not duffers, won't drown. If duffers, better drowned."
competent designer
It's a child. How competent can a child be?
I was the younger brother. I was "willing" to do a lot of things I knew were bad ideas.
My younger brother tried out all of my inventions. He was "willing", which really meant he never knew what I was asking him to do until it was too late.
I definitely remember hearing "Here put this helmet and [thick] jacket on"... "Stay there and wait" etc
I was an only child so I had to do both.
You're still here so, good job! 👍