If they believe the numbers then I'm inclined to trust the United Nations and the BBC, but just in case, here's some fact checking from The Guardian

https://www.theguardian.com/world/2023/oct/26/can-we-trust-c...

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Omar Shakir, the Israel and Palestine director at Human Rights Watch, said he saw no evidence that the numbers were being manipulated.

“We have been monitoring human rights abuses in the Gaza Strip for three decades, including several rounds of hostilities. We’ve generally found the data that comes out of the ministry of health to be reliable,” he said.

“When we have done our own independent investigations around particular strikes, and we’ve compared those figures against those from the health ministry, there haven’t been major deviations.

“Their numbers generally are consistent with what we’re seeing on the ground in recent days. There have been hundreds of airstrikes per day in one of the most densely populated areas of the world.

“We’ve looked at satellite imagery. We’ve seen the number of buildings, and the numbers that are coming out are in line with what we would expect with what we’re seeing on the ground. So you put all those things together and we’re quite confident in the overall casualty numbers.”

That article does not support the 13,000 number that is being bandied around in the past few days. The figure discussed in that piece is much lower, and a former Reuters bureau chief is quoted to say that news organizations should be sceptical of any numbers originating from Hamas-run sources.

That article says the casualty numbers coming out of Gaza are generally legitimate, and it says ~3,000 children were killed as of October 2023.

For a March 2024 casualty update, ~13,000 children killed seems, sadly, plausible.

Yes, you should condemn Hamas for putting them in peril! They just need to keep their soldiers and weapons away from the children instead of using them as meat shields and banking on their supporters like you to help them with the information war.