R.,
I would try to learn a few words/phrases in Mandarin to help them to feel more at home. I know this works, from speaking the little Japanese I know to a child in my school, and French to a French-speaking child, too. In both cases, the child felt more at home with me because of my (albeit feeble) attempts to speak their primary language.
One easy way for you to learn some simple phrases, and to bond over the experience, is to watch a kids' show, like "Ni Hao Kai-Lan"... yes, you won't learn to read the newspaper in Chinese overnight, but you can communicate some simple words. :)
In terms of meals, rice is definitely a staple (the kind that is usually cooked in a rice cooker - jasmine or other slightly sticky rice) in an Asian diet. Simple stir-fries are good (a little chicken or beef or pork with broccoli, tofu, etc.) and dumplings are a good staple (which you can buy frozen in Asian food markets, and then just fry/boil them when you want to eat them. Chopsticks are nice to have around. I say this, because I am Asian-American, too, and I love to use chopsticks for most of my meals. :)
Usually, dessert is simple, and can be ice cream/jello/ fruit.
Basically, your new foster children will need the same thing any other foster child would need: lots of love and reassurance and a place to feel cared for and safe. Even if you can't find all things Chinese, you can definitely provide a warm and loving environment for these girls. I wish you much luck. XO.
p.s. - One thing you might consider (if you haven't already) is that the girls might want to co-sleep/share a room because the situation will be so new to them... I know that even when my parents left me and my sister with our grandparents for a week without them, we preferred to be together in the same bed. And we cried about missing our parents, even though we were with our grandparents!! So, maybe expect some difficulties in the beginning, but you sound like a very caring and wonderful person, so I am sure you will adjust and give a great home to these children. Best to you.