I've already looked up this page. https://parisbytrain.com/navigo-pass-prices/
But I can't find the prices of Navigo Pass for child?
Do you offer the Navigo Pass Prices for child?
How much does it cost for 9 years old boy buying a Navigo Pass (Week price)?
Navigo pass doesn't have a discount prices for visiting children unfortunately. French resident school children use Navigo passes with an Imagine R annual subscription and those are discounted, but these are not available to visiting non-resident kids, plus it would take several weeks just for the application & delivery.
If you happen to be visiting over a Saturday, Sunday or French public holiday day, your child can use discounted Paris day passes known as Ticket Jeunes which are available at ticket vending machines and from ticket windows. Ticket Jeunes day pass cost is 3.85€ for 1-3 zone and 8.35€ for all zones for a day. Ticket Jeunes validity is from first usage during a day, up until midnight that same day, not a contiguous 24 hour period. Ticket Jeunes day pass is open to anyone under 26 during weekends & holidays, NOTE! Ticket Jeunes are not valid for Orly / CDG airport travel using the RER/Orlybus/Roissybus.
I'm guessing you've already noted that RER tickets are discounted for children 9 and under so you can take advantage of single RER ticket discounts.