"The surge has also led to a sharp rise in pediatric cases. For the week ending Jan. 6, more than 580,000 child coronavirus cases were reported, marking a 78% increase from the week ending Dec. 30, according to the last updated data from American Academy of Pediatrics."
Kids can get 'Long COVID' and boosters among older adults shows much improved immunity from infection, so it would be reasonable for kids to benefit in the same way...for their health, and lower risk of spreading the virus to others.
The U.S. also has an 'embarrassment' of vaccines, which the WHO recognizes and encourages us to share...could be another reason for their statement...not a bad thing on their part, btw...I'd do the same in their shoes.
In answer to your question...Yes, the WHO is indeed credible, but this is not a "Yes/No" sort of issue...as noted above.