Hi OP...
This is a good turn of events. DON'T QUESTION why she's asking you to do it. Just go with it in a very matter-of-fact manner. Do as you're told and don't read anything into it. I wholeheartedly agree with the guy who suggests you ask the 7 yr old if "this is how mommy washes you". He's right- the girl will spill if your GF does anything 'different'.
Keep us posted on any more comments or little "hints" the GF drops. It's definitely too soon to make a determination yet. However, at the very least, she's signaling she trusts you with her daughters. That's good for your relationship with her... anything else is premature.
Play it cool... don't question. Just bide your time and let us know if any other hints come out.
= The Guru =