In order to delete old backups from your Windows PC, you will need to delete the backups manually from the file they're saved to.
If you have decided to use iMazing's default file file folder, the backups will be located in either one of these folders:
C:\Users\*Your PC's name*\AppData\Roaming\Apple Computer\MobileSync\Backup
C:\Users\*Your PC's name*\AppData\Roaming\iMazing\Backups
You can determine which folder your backups have been saved to by going into iMazing and, right-clicking on your child's device and choosing "Options" in the menu that pops up. From the page that opens up, click on "change" next to Backup Location.
After finding which folder your backups have been saved to, open your File Explorer. You can copy and paste from above or manually type in the file folder your backups are located in into the directory bar and then press "Enter".
From the Backup folder, you can delete all of the backups that are shown there which will clear your hard drive of the memory of those old backups. Keep in mind that if you're deleting these backups to free space on your hard drive, you will need to empty your Recycle Bin as well.