Controlling mobile costs of kids

Hi,

I gave 1 mobile number (Pay&Go) for each of my 2 kids and I would like to have an easy way to control their usage. Can I add their numbers to MyProximus account? How? Or should I do it in some other way? They don't have a "normal" phone, so they can't call #121# or install the MyProximus app. The ideal way is through my laptop.

Thanks,
Joaquim