Add Proteus File as Subcircuit
As I can remember proteus can copy the complete subcircuit and past it anywhere else including the child sheet but if it doesnt work you can simply use a feature of proteus called "Project Clip", this works by selecting the complete circuit in Isis and the complete circuit in Ares, the you go to menu File > Export Project Clip, this operation saves the circuit tha you want to integrate in the complete design even with the component disposition in the PCB.

After you`ve exported your project clip then do the operation menu File > Import Project Clip into the new design, this operation will put your saved subcircuit in exactly the same disposition in what was saved for both schematic in and pcb, in ares your recently  imported Project Clip (all the component) will be grouped and at the first component in the list you'll have a scissor, just put those components in the usual way, selecting one of them will select all the imported components.

If you need more explanation go to the Proteus help and search for the Project Clips help.

I hope I could help you.