S.H.
14 months old is young.
the 2 and 4 year old can get it.
All I did was... used 2 paper plates. On one I drew a happy face. On the other a sad face. For each one, per my kids' behavior, I would make a tally mark. It also teaches them how to count.
Then I would explain it.
They liked it.
It worked for a time. Then kids get desensitized to it. So then you have to try something else.
Marbles in a jar. taking one away with each infraction etc.
It does not have to be complicated. The easier the better.
Bear in mind, that kids under 5 years old, do not yet have FULL impulse-control developed yet. But they do understand... mentally.
Your kids are young.... they do not yet read... so, anything used should be visual... and EASY SIMPLE to understand.
Just make your own.
I did not and do not... give treats or 'rewards.' Otherwise a child will then EXPECT (because they have been taught this), that every time they do something nice, they are supposed to get a treat. That is not the lesson in it. For me, I just give praise, a high-five... a hug. I tell my kids "good job."
Once in awhile for exceptional things... which they do ON their OWN volition... I do, reward them with something. But not always materially.
all the best,
Susan