Welcome.
These are midi controllers with no built-in sounds of their own. You have to plug them into a computer running a virtual synth app like Arturia Analog Lab (which the controller already comes with), or into an iPad running iMini, iSEM or iProphet.
If you want something simple for your kids to learn to play on with minimum fuss, buy them a cheap electronic piano/keyboard. All but the most inexpensive keyboards and electronic pianos usually have a USB socket on the back, allowing the unit to be plugged into a computer or tablet and used as a basic midi controller, should you or your children decide you want to explore that avenue. But check before purchase that the keyboard definitely does have a USB socket if you think that will be important to you.
