But the Tigers' leaders blame the recruitment on ill-disciplined junior members of the organisation.
The Norwegian monitors say they raised the issue in fresh talks on Tuesday.
Last week the monitors said the Tigers had recruited more than 300 children in the previous year, in spite of a ceasefire being in force.
Trouble makers 'removed'
The monitors "raised concerns on abduction, harassment and child recruitment," spokesman Teitur Torkelsson said, the AFP news agency reports.
"[The Tigers] took the matter very seriously and said the main reason for this is the indiscipline among some of its lower ranks."
Mr Torkelsson indicated that the Tigers' leaders would act against those of its members still violating the ceasefire by recruiting under-age fighters.
"In the past they have removed some of the trouble makers after it was brought to the attention of the [Tigers'] leadership," he said.
Sri Lankan President Chandrika Kumaratunga says the Tigers are continuing to build up their forces, despite the ceasefire which came into force last February.
And she disputes the monitors' figure of only 300 children having been taken on.
A presidential spokesman last week put the figure at more than 10,000.
Overall, the Norwegian monitors have been positive about the issue, saying that the number of complaints of child recruitment have been falling substantially.
February's ceasefire ushered in the longest period of calm in Sri Lanka in two decades of fighting.