The Tigers said on their website that all the children joined them voluntarily in Batticaloa, a town 230 km east of the capital Colombo.
It said five children were returned to their parents and another 15, who refused to go home, were sent to a Tiger-run skills development center.
Human rights groups have accused the Tigers of using about 4,000 child soldiers.
The rebels began fighting the government in 1983 to create a separate homeland for Sri Lanka's ethnic minority Tamils, claiming discrimination by the Sinhalese majority.
The Tigers claim they have returned 1,445 children to their parents.