These pictures from an Islamic Jihad "Revenge of the Free" training camp are not ambiguous.
This is a war crime.
As the ICRC says,
The Optional Protocol to the Convention on the Rights of the Child differentiates between States and non-State armed groups in setting the age-limit for recruitment and use in hostilities. For States, the age limit for direct participation in hostilities and for compulsory recruitment is 18. This means they can accept voluntary enlistment of persons between the ages of 15 and 18. Armed groups, on the other hand, are bound by a stricter prohibition, affecting both voluntary and compulsory recruitment of under-18s.
There are no photos of the kids engaging in sports, playing games or otherwise having fun. Calling this a "summer camp" is a joke. It is a military training camp for kids. And Islamic Jihad makes this clear in their recruitment video:
Not only that, but Islamic Jihad freely admits that the targets of the weapons the children learn to use are Jews. Islamic Jihad official Darwish al-Gharabli said, "These camps qualify the generation to carry the banner after this generation, part of which has been martyred; it also establishes a generation that is aligned with the path of jihad and resistance; believing in this option and that Palestine is the central issue and fighting the Jews is an act of worship."
The UN and its agencies, and Defense for Children International Palestine, and other "human rights" NGOs are curiously uninterested in this incitement to violence and these violations of children's human rights and international law