Alexandra,
flashlight / water as star and atmosphere makes a lovely demonstration that can be a prompt for further investigation. You’ve planned for the children to wonder and explore by writing what they think is going on. Then their questions could pose a starter question for an investigation.
Does the size of the jar make a difference? Does a ‘star’ twinkle more if it goes through more ‘atmosphere’ ?
{Yes, in the case of real stars, which is why Sirius, the brightest star, which is always quite near the horizon from Ireland, looks so twinkly}