Instead of using transformer throttles, why not use push button controls mounted on the side of the layout? Wire them to an insulated track section on each independent loop, with each loop wired to dedicated power supply/transformer(s) that you can set to the desired the voltage behind the scenes to control the speed. The locomotives would start and stop on the insulated track section when kids (and adults) push the buttons; it just temporarily energizes the isolated track section to get the lomotive started until it hits the dedicated power on the loop it's running on. Train goes a full loop around the track and then stops again on the insulated track section until the button is pressed again, or will keep running if power is kept on the isolated track section by keeping the button pressed.
Pretty simple, basically simple on/off control and easy to keep wiring hidden. You could also wire up separate sound activation buttons for whistle/horn for each loop as well that feature is desired.