When your son went inside the machine he may have dislodged the power switch lead.
Basically the power switch on the case just signals the motherboard to start, it does not actually supply power to the PC.
On the motherboard there will be a buch of connectors for HDD LED, speaker, reset, power switch etc. It's probably worth making sure these are all still connected.
As for the password. There is often a jumper setting on the motherboard to clear the BIOS settings. This would remove the password, and set the BIOS to default. Otherwise, removing the battery is the right thing to do. Leave it out for some time and then start the machine without it in. This may also work.
Give those thinks a try first off.
Nick