Сейчас is like English "right now", something is happening at this moment in time. Теперь is used more to contrast with another point in time, things were one way before, but they're different now. In this sentence, rather than meaning you need to do this action right now, the English suggests that something, probably bad, has happened, which now needs to be explained to Mom, as opposed to before when it hadn't happened, so теперь is used. There's better explanations than that around, if you search discussion.
Теперь is a combination of этот /that/ + первый /first/ and has a function similar to 'hereafter' in English. In contrast, as already mentioned by a previous comment, сейчас (сей /here, now/ + час /time, hour/) means exactly 'at present', 'right now' and bears more narrower sense.