I haven't managed to wrestle the tablet back from the kids yet, but managed to install Polarr on my Chromebook and...it's excellent! I'm only using SOOC JPGs as the laptop is a bit lightweight in the CPU department, but the processing files is easy, with many of the necessary tools available in the free version.
I just edited a couple of photos in no time...the first one resized down to 1024 pixels wide and around 300kb in size for upload here (wanted to see what sort of quality was produced)
bunnies
pier
These are just snapshots pulled at random from my camera - not meant to be serious shots!
First shot had rotation applied, highlights reduced and a small amount of sharpening. The second one had a filter applied, blacks reduced, grain added and a small amount of sharpening.