After 16 years of calling dances for children, I've learned a few things, and I've learned most of them the hard way.
First, let kids pick their own partners. No mandatory boy/girl pairings. Avoid that drama all together.
Second, keep mixers to a minimum if you do any at all.
Third, keep it simple and fun. Dances that you think are simple can be too complicated for a kindergartner.
The following two dances are simplified versions of two classics, Patty Cake Polka and Ninepin. I try to write my directions using musical phrasing, but don't worry if they get off the music.
Patty Cake Promenade
Formation: Whole Set, Gender free, Begin by taking two hands with partner
A1: With the foot closer to the music, heel/toe heel/toe
Slide 3 steps up toward the music
With foot farther from the music, heel/toe heel/toe
Slide 3 steps down the hall
A2: Clap right hands w/ partner 3x
Clap left hands with partner 3x
Clap both hands with partner 3x
Clap your knees 3x
B1: Do a two-hand turn with partner
Make a bridge
Top couple ducks under the bridge to bottom of set
Everyone move up
Formation: 4 couples in a square with one extra person in the middle, aka the duckpin.
A1 Head couples sashay across the set w/ partner (passing on the right of the duckpin)
Sashay back on the other side
A2 Side couples sashay across the set w/ partner
B1 & 2 Duckpin takes hands with couple 1 and all three circle to the left.
Duckpin moves on and repeats with couples 2, 3, and 4 in turn.
Duckpin takes the place of a dancer in the square and the dance begins again with a new duckpin.
*I came up with the name because of my Baltimore heritage and because duckpins are smaller than ninepins, perfect for a kid's dance.