Hi,
In order to resolve this what you can do is use " Layers". Put the composition in the top most layer of your Muse file and you will be able to see it appear on top of the contents of your child pages.
Please refer to this screenshot :-http://prntscr.com/4bxc6b
Also, You can learn more about layers using this link:-Working with layers in Muse CC | Learn Adobe Muse CC | Adobe TV
Cheers,
Rohit Nair