Kids Bluetooth Headphone Volume Limit mod
Adding resistance in parallel will not attenuate the signal. It will draw more current from the amplifier output, however, and if the resistance is low enough will cause current limiting. This would be likely to cause audible distortion and generate harmonics which would be detrimental to young ears.

Series resistance is the way to go but due to the logarithmic response of the human ear you will be surprised at the attenuation factor required to achieve the desired reduction in volume. The dB, decibel, scale takes this into considaration and each 3 dB reduction in volume results in a halving of power.

I suggest you play around with resistor values to obtain the desired result. Start with series resistance equal to the speaker resistance and if you require further attenuation then double the resistance each time.