Serviio

I have just bought a new tablet for my son for Christmas, so getting it ready I went to download Serviio; however, it cannot be found anywhere, is there a reason for this? even with the devices that already have Serviio installed it still doesn't appear in the Playstore

Serviio is not for tablets. It is the media server side, not playback. You install it on a computer or NAS, and then stream to any DLNA compliant player, which you can install on your tablet.

Your tablet will have many choices in the play store for players. Since I think you're referring to an Android tablet, I know there are many available. VLC is a very good one, and there are many others, both free and paid.