"Generally the larger, 14-point font size was considered to be easier
and quicker to read, as well as being more attractive and more desired
to be used in schoolbooks. In addition, the Comic font type was
perceived as being easier to read and more attractive, as well as
being more desired to be used in schoolbooks than the other font
types. Along with Arial, it was also preferred over the serif fonts
for use in schoolbooks. Overall the 14-point Arial and the 12-point
Comic was the most preferred font types."
You can read more here : Which Fonts Do Children Prefer to Read Online?
"Early primary years:
Use text redundantly with images so that pre-literate users can access
Use simple text.
Use fonts that approximate how children learn to write. For example,
many fonts use “a” and “q” in variants that do not match how some
children are taught to write those letters.
Do not use dialog boxes.
Don’t require explicit “save” operations. Save work automatically.
Exclude extraneous content.
Provide highly interactive and engaging applications.
Avoid visually noisy interfaces – they are distracting.
Provide large target areas.
Allow children to personalize.
If applications will be used on a smartboard, do not use a footer that
can be accidentally activated by children leaning against the surface.
Support cooperative use, with two or more children using your product
at the same time.
Design to support teachers and parents or guardians, who are likely to
be assisting or supervising usage."
You can read more here: Designing for Children
"When selecting a typeface for a children’s text, look for a warm,
friendly design with simple, generous letter shapes."
You can read more here: Typography for Children