My brother's neighbors are fluent in French and English (the wife is from Quebec, and since she's lived all over the world, she's actually multi lingual). Both her kids (maybe 3 and almost 2?) understand both French and English, and the oldest can reply back in either easily (I met the younger one before she was really of speaking age). She also runs a daycare and all the kids have picked up on basic commands and greetings/courtesies in French. As for consistency, she seemed to switch back and forth a lot (even mid-sentence), and there didn't seem to be any confusion with the kids.
Although when she tried to throw Spanish in there, the oldest said "Moooom, I already have English, French and Science. I don't need Spanish now."