The link you posted in the comment says it all.
All you have to do first is to root the devices and then make the your apps System apps using (ES file explorer, Titanium backup, /system/app mover ★ ROOT ★ ...) and then unroot the devices.
Also you can you a parental control app like (Norton Family Parental Control , Net Nanny , Qustodio Parental Control , Mobicip ....) check this link.
As for a legal and secure way you can always contact the company and ask to include your apps as system apps.
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solved Protected Android industry app [closed]