Sorry to hear that!
The mBot Ranger is a high power robot which consume the power in the battery fast. So, from your description, the robot can’t get enough power from the battery.
According to our test and research, it is suggested to use rechargeable Li-ion battery or rechargeable nickel-metal hydride, nickel-cadmium which can be bought from amazon or local shop. Or Alkaline battery with good quality like Energizer, DURACELL (but non-rechargeable battery can’t last long).
Sorry for the trouble. You are a loving father, hope you and your daughter have a most happy and prosperous New Year!