His blood be on us and our children
Here’s what it means
We accept responsibility for the consequences of this death.
Why the Bible says it this way
The crowd uses a legal accountability formula, not a perpetual authorization for hostility against later Jewish people.
An idiom is a familiar phrase whose meaning comes from the whole expression, not from reading each word by itself.
A helpful detail: This verse must not be used to assign collective guilt to Jewish people across history.