It is common practise to use a jack that has a "normally closed" set of contacts, wired so that when no plug is inserted into the jack, the audio is connected to the built-in speaker. With the plug inserted, the internal speaker is disconnected, and the audio is fed to the plug, and the other end of the plug's cord is hooked to either an external speaker or headphones or whatever...
For solid-state appliances, no terminating resistor is needed.
Is the input to the Guitar Amp LINE level (~1V) or PICKUP level (a few mV)? If the second, then the Speaker level out of the toy will greatly overdrive the guitar amp and sound horribly distorted. Wont hurt anything, but it will sound awful.
Try an attenuator consisting of a 1kΩ and a 100Ω resistor in series, with the speaker signal to the 1k and the 100Ω going to ground. Connect the amp input between the junction of the two resistors and ground.
Edit: Or connect the CW terminal of a 1kΩ pot to the speaker signal and the pot CCW terminal to ground. Connect the amp input between the pot wiper output and ground.