How do I adjust my child's bike shown in this video to improve fit and comfort?

Timeline

Provided because I learnt a lot in the process

1. My 5yo daughter has started growing out of her Apollo Petal with 14″ wheels and an upright position typical of a child bike.
2. Basic research and measurements lead to the conclusion she might be ok with a bike with either 16″ or 18″ wheels.
3. I found a good deal on a used mountain bike with 18″ wheels so we tried that first but it was clearly too large for her.
4. I bought her a Decathlon B’TWIN 500 SPY HERO (although the sticker on hers says 900?!) on 16″ wheels, still with a fairly upright position typical of a child bike.

The problem

I’m struggling to adjust it so that it’s comfortable for her! Neither of us have the patience for more trial and error, so I’m hoping someone with a bit more experience can tell straight away what the issue is.

1. A similar bike has a good product video where the actress
   seems to have the bike adjusted exactly right (link is to product
   website where video is included among photos, I don’t think it’s
   possible to link to video directly and I couldn’t find it on youtube
   or similar)
2. This is what my daughter looks like on her bike at the moment. Is she just still a little short for it, or is there something I can do to improve her fit?

Things to note:

• The seat only has height adjustment (I can’t tilt it or move it forwards/backwards)
• I am reluctant to put the seat higher up. At the current height, her
  heels are slightly off the ground when sat.
• The stem and handlebars are integrated so also only have height
  adjustment too. They are nearly at the highest setting. We tried having them lower and it was even more awkward, she was constantly standing up on the pedals.