I think it's possible that the real constraint you face is not the number of players, but who they are. Your daughter is new to the game, and if I read correctly between the lines, her mother is probably not a hard core D&D player. Is that accurate?
If so, I suggest that you start with combats that are easy to the point of trivial. Watch their reactions; their expectations may be completely different from what you anticipate. If they don't need Level + 3 encounters to have fun, don't bother with them.
Also, you might encourage them to really get into the roleplaying of the combats -- the swings and misses, the jumping on tables and charging about. They might get a lot more satisfaction out of that -- taking their time to have fun with it -- than from a much harder encounter that really challenged them.
Of course, they might both in time develop into seasoned tactical CharOp fanatics -- you never know.