Good point, Cory. I knew an English family, living in Germany. The son went to work with his Dad one day and heard him speaking German to his colleauges. From that day on he refused to speak English with his father as he "didn't know you spoke German, Daddy". He was 6yo and never spoke English again with his father, he felt betrayed.
How about getting Oma to get some familiar books in German and read them. The Gruffallo books are translated into German and good fun. She will have to say the words in German then in English to make sure they understand.
Don't make the mistake of thinking that when you start singing nursery rhymes to them and they sing them back that they actually understand what they are singing. I remember singing Happy Birthday To You with a group of kindergarten children and asking them what it meant. None of them knew, but they all could sing the song word for word.
She could start by doing games eg. the children dance to music, Oma shouts "Hinsetzen" and the first to sit down is the winner, then she shouts "Aufstehen" and they have to stand up again. Kind of a German musical bumps.
Get her to teach basics such as run, jump, touch, hop, stand up, sit down, fall down, by doing the actions with them. She can then add colours, the body etc.
Another good game for colours is to shout the colour and the children have to find something in the room that colour. She repeats "Etwas blaues anfassen, blau, blau, fass etwas blaues an..."
Lots of repetition, not too much at once.
Once they are more confident then you could see about introducing TV or DVD. You can get German TV in UK but only with satellite TV afaik. There are some good DVDs though. And look out for the CDs of a German guy called Rolf Zuckowski, a rare find: Childrens's CDs that adults like to listen to as well.
Good luck!