erm [boggles at nonsensical OP]
You're saying you've kept your child off but you think it's pointless - well, no, it's not.
It's very likely that there will be people in school who were nOT exposed to your child's infection last week. And to send the child in now would be putting them at risk. How don't you understand that?
If a teacher is pregnant, are they supposed to stay off school for the duration in case a child comes in who is infectious and puts an end to their pregnancy?
You wouldn't know if they were pregnant or not.
The whole point is, yes, it's possible the child infected others before you knew they had anything. And now it's possible they will still infect others, and you know they might, so you're keeping them off school.
That's the right and only thing to do.