"There are people on this thread saying that 10 year old boys cannot be trusted not to abuse 5 year old girls."
There are people on this thread saying that some 10 year old boys can't be trusted not to hurt 5 year old girls.
I can't believe you genuinely think otherwise.
This boy is a stranger to the OP. He is not well known to her ex or her daughter.
So of course she can't trust him.
That doesn't mean he's not trustworthy.
Or that he's not a wonderful kid.
Just that it's normal to worry about your 5 year old kid sharing a bed with a ten year old boy she doesn't know well.
Most parents would worry about the child spending all night alone in bed with a much older child they didn't know well, regardless of gender.
Kids can be really horrible to one another.
And saying that does not imply that all children are horrible to all other children and need to be kept in isolation.