Level
There are two ways to play chords. All fingerings on this page are meant for your left hand.
The first way is to use your pinky, middle finger and thumb, so 5 - 3 -1. This way your hand stays within a group of 5 keys, which is very pleasant for most children. The disadvantage of this fingering is that your hand does not feel very relaxed. Because you always have to play the bottom key with your (short) pinky. Try to use the tip of your pinky, though that's rather difficult. It's better than playing with the side of your little finger.
The second way is to use your ring finger, index finger and thumb, so 4 - 2 - 1. This fingering is often a bit confusing at first. The advantage is that you can relax your hand more easily and can reach the keys better. Anyhow, choose the fingering that you suits you best. And if after a while you want to use the other fingering, that's perfectly fine too! It's always good to be able to use both fingerings.