Am I missing something really obvious?
Knowing that the answer is combination "6" in the example doesn't really help:
Opening mother to reveal another red means that the baby MUST be in one of the other sets (as it's the only set with 2 consecutive reds) - no way of knowing which one.
Opening mother to reveal green means the baby MAY be in this set, but equally could be in the other set which starts Red Green - again no way of knowing which one.