Once your suspicions have been aroused make an appointment to see an optometrist and obtain a formal diagnosis. Most optometrists will provide a free eye test for children. All children should have regular eye tests anyway, but make sure you ask specifically for colour blindness to be tested. Some optometrists do include a colour blindness test as part of their free eye test for children but most national chains do not, so always make sure you ask for a colour vision test specifically in advance and if there is a charge for this go somewhere else! Ask which test the optometrist will use, this will usually be the Ishihara test.
It is important that the specific type of colour blind condition is diagnosed professionally because (i) support provided in school can be tailored to suit each specific condition and (ii) the type of colour blind condition someone has may affect their ability to pursue certain careers.
Don’t rely on online tests, these are not accurate!