Several possibilities there:
You can ask the child in a cutesy tone (Duzen), if it's a rather young child. The mother will probably smile from ear to ear and wait for the kid to answer, or eventually answer the question for her kid (or call the police for harassment ;)
If the kid is your friend (similar age, same school, etc. etc.), you can just quickly greet the mother (Siezen) and talk to your friend as usual.
If you only want to talk to the mother, you would address her with "Sie" and pretty much ignore the child.
So basically, you address everyone individually as appropriate. If you want to talk to the group as a whole and there is a clear difference in authority (such as parent and child), you usually talk to the person "in charge" according to the usual rules of Duzen/Siezen.
If you would address each person in the group the same as an individual, then addressing the group that way is appropriate as well.
Examples: You would usually "siezen" a traveling group of pensioners, and usually "duzen" a group of teens or younger, depending on circumstances.