The New Israel Fund writes, "Between 2017-2019, five thousand children aged 12-18 from the West Bank and East Jerusalem were arrested by the Israeli military and tried in military courts, including dozens of children under the age of 13."
No source is given.
The number is absurd. It would mean that Israel averages between 4 and 5 arrests of children every day, which would be about 32 every week for that three year period.
According to the UN's OCHA-OPT, looking at a random sample of their biweekly Protection of Civilians reports, there are far fewer children arrested - 8 in this two week period, 13 in this biweekly period, and usually none mentioned.
But NIF goes beyond that, claiming not only that they are arrested but that they are tried in military courts.
A favorite statistic from many NIF grantees comes from a Haaretz article in 2011 that said that Israel has a 99.74% conviction rate in military courts in the West Bank.
If NIF's and Haaretz' numbers are correct, there would be thousands of children in prison! However, there are only a couple of hundred, nearly all of them males over 16.
Even other anti-Israel NGOs, like Defense for Children International, says that Israel arrests between 500 and 700 children a year - far less than NIF's claim of over 1300 a year, and they do not even pretend to say that every one of those get tried. (They also do not say the source of that statistic. UNICEF in 2013 gave a similar figure, but it admitted it was a guess based on speaking with lawyers in the courts who defend the children.)
The NIF statistic is made up from whole cloth. And these are the people who fund lots of other left-wing groups critical of Israel like Breaking the Silence and Yesh Din.
I suppose that NIF doesn't request the truth from the groups it funds, either.
(h/t Tomer Ilan)
UPDATE: Tomer found a HaMoked report where roughly 3000 Palestinian kids were detained in 2018-2019, but there is a big difference between "detained" by police and "arrested and tried."