A stencil would work better than freehanding it with tape. But you might want to look at wallpaper (even removable wallpaper would work). It will be far easier to apply and it will take less time. It will also be easier to change out when she gets sick of it (as kids always do) in a few years.

Removable wallpaper!
https://www.etsy.com/listing/525212371/black-triangles-for-the-office-vinyl?gpla=1&gao=1&&utm_source=google&utm_medium=cpc&utm_campaign=shopping_us_c-home_and_living-home_decor-wall_decor-wallpaper&utm_custom1=9875a7af-5494-49aa-b7ea-1a96920cd601&gclid=EAIaIQobChMI3quWnrfY2QIVjYjICh37xAs6EAkYByABEgK77fD_BwE

Thank you! I had looked into wallpapers but I was hoping to spend less than $400 for one wall.

Checking out cutting edge stencils website, search under geometric maybe she can find a design that you could use a stencil to duplicate.

Stencils will work you can even make your own with an exacto and mylar . To figure it out for the wall use some graph paper and be prepaerd to spend a Sunday with the puzzle.IMO what makes that wall interesting is the center triangle.

So the link to Etsy allows you to custom order the removable wallpaper, which might mean you can order it without the small triangle in your own color