What kind of tree is it? It may be getting too much water. Or, have you moved it recently? If you move some trees like ficus (that looks like a ficus microcarp which is what I have) they have a "drop leaves spell". If that is the case, it will snap out of it in a couple of days to a week when it gets used to its environment. Try telling me all of what you, or your daughter, has done to it in the last week or so. That will help me determine what is wrong with it.

Good luck!

Woops! It's microcarpa! Not microcarp!

I might suggest the chopstick method in regards to watering.
It sounds to me like an overwatering situation.It is an easy
thing to do with Bonsai. You get a chopstick, stick it in the soil, if it comes out with moist soil attached to the stick you're plant does NOT need water, if it comes out clean it is time to water.Water so that it seeps through the drain holes in the pot and don't water again until
you test it with you're chopstick and it comes out dry.The idea being, you water when the plant needs it, not according to you're schedule (ie: I always water on Wednesday for ex.)If it were me, I would cut back on the fert until the plant shows prominent signs of new growth recovery all over. I have my Bonsai under a grow light also
16hrs on and 8 hrs off. I hope that helps - sorry if I ran over at the mouth too much!
swanny