'Awesome' and 'Chunk Error' are good example of seeds that you can use for Minecraft PE. There is no seed that will give you an exact super flat world, but you can use the closest seed, which is 'Piggy'. It will give you a level beach that has few strands of grass.
Well I have a bunch of seeds creativemodesucks gimmedimionds stupedfish/stupid fish helloooladies nyan easybib jpgaming raygun OneDirection<3 bigbigbig 0.4.0 CaptainSparklez ONEDIRECTION dumbocow DonkeyDong TurnOfTheTides bambi Mountainman 1497848687 909090 Doom Rooster Friday Cheppa Flattering Flat world Iliketomoveit jhonny golden apple biom line Minceraft herobrinewar USMC flatworld Flatland football _1/"_1" outcrop 44373107782 Sandyworldsandbiome Gothamburg 1350017340 Dinosaurs avgn infinity flat plzz blank ********** SuperMario3DLand subscribeordie jaymzisawesome hunter 404 Horrid Seed windy jimmy lavaLaVaLA WATERFALL!!! leaveblackforaramdomseed epic the lorax Hope you like them :)