Now you talk about earphones instead of in your first post.
Earphones are not connected to an input, instead they are connected to an output. You are connecting the jack to a speaker output so when you plug in, then the speaker will be disconnected by the switch on the jack and normal but a little too loud sounds will be fed to the earphones. I hope you do not have earphones with a 4-connections plug that is used for a cell phone. Instead you probably have stereo earphones that will play in only one ear when plugged into a mono jack.
I thought that since the tiny speaker on the toy was hard to hear in the classroom then you were feeding the output signal from the toy to an amplifier microphone input on an audio system.