On your sister's site, in the public_html folder create a sub-directory called kids. Then put the files for the kids site in this folder. I think this is what you're asking for.
An alternative is to log into your sister's Control Panel, select Sub Domains and create a sub-domain called kids. This will create the sub-directory mentioned above where you can place the kids files. Then you can access the kids site via either of the following:
http://www.janetemarshall.co.nz/kids
http://kids.janetemarshall.co.nz
The advantage of using a Sub Domain is that your sister can then create e-mail account such as:
FirstChildsName
janetemarshall.co.nz
SecondChildsname
janetemarsha.co.nz
...