I believe the most basic instrument to start with is the keyboard or piano. It is very hard to learn other instrument without the basics. For eg. keyboard have their own pitch ready to be heard. Comparing to violin, it is so difficult to capture the C, D, E, because you need to press your own pitch/tone/key.

No use learning theory without the instrument. waste of time honestly. children like it when they can hear sounds. otherwise, why do some parents buy simple keyboard for their children to knock away when they're like 1-2 years old. if they knock it, means they like it, right.

hope this helps.