Great question! The common alignment pattern for young children is to stand with their bellies poofed out in a swayback posture. In that position, the hip flexors are in the shortened position, and the hip extensors are in an elongated position.
It's also not uncommon to see a young dancer tucking their pelvis. Both patterns limit the mobility and the move-ability of the dancer. Here are some ways to help them find a neutral pelvis.
One trick is to have them stand at a wall, facing away, with their pelvis lightly touching the wall. Their shoulders should not touch at all—even the skinniest of dancers have some pelvic protrusion. When they do their demi-pliés, they'll keep the gluteals lightly touching the wall as they slide down, keeping the weight even on the three points of their feet, and without the shoulders touching the wall. You might give them the image of a merry-go-round horse and sliding up and down on the pole.
Have them lie on the ground, knees bent and arms by their sides, and do slow bridges, maybe even holding at the top. This helps strengthen the extensors as well as work the core. Make sure they keep breathing!
Then have them stand back up and imagine themselves like a flexible tube, getting long and skinny like Gumby. That should help them find the vertical line of the body, instead of lifting their ribs as a way of standing up straight.
Slowly they will learn that a neutral pelvis is having the pelvic bowl upright and aligned over the legs.