Having enough to say
If your child is still stuck, find out if they have read up enough on the topic. Some inertia with writing can be due to lack of knowledge. They will find writing so much easier if they spend another day or two reading more on the topic and gleaning some additional ideas.
Try using a neutral sentence
Suggest starting the essay writer with a neutral sentence: a sentence that merely states an interesting fact on the topic being written about. Here’s one: ‘Mozart was one of the most important Austrian composers of the eighteenth century.’ First sentences in essays don’t need to be stellar - you just need to start!
Now, go write that essay!