The statement appears to be entirely accurate.
https://www.unicef.org/press-releases/least-10-children-kill...
> In 2024 alone, at least 600 children have been killed or injured in Myanmar.
https://www.unicef.org/press-releases/geneva-palais-briefing...
> As feared and as warned, the situation in Sudan has become fatal for a frighteningly large number of children. In reports UNICEF has received that cover the period from the start of the fighting until 25 April 2023, 190 children have been killed.
https://www.unicef.org/press-releases/unimaginable-horrors-m...
> In total, more than 50,000 children have reportedly been killed or injured since October 2023.
Mariupol.
https://en.wikipedia.org/wiki/Siege_of_Mariupol
> Ukrainian officials reported that approximately 25,000 civilians had been killed…
Even if every one of those 25k were kids, still no.
The actual report "Save the Children" cites is available at https://digitallibrary.un.org/record/4084012?ln=en&v=pdf - page three of the English report states:
> The violations verified in the greatest numbers were the killing (4,676) and maiming (7,291) of 11,967 children, the denial of humanitarian access (7,906 incidents), the recruitment and use of children (7,402) and the abduction of children (4,573). The number of children detained for actual or alleged association with armed groups including those which are currently under sanctions enacted by the Security Council, or for national security reasons, surged to 3,018, further depriving children of their rights. The highest numbers of grave violations were verified in Israel and the Occupied Palestinian Territory (8,554), the Democratic Republic of the Congo (4,043), Somalia (2,568), Nigeria (2,436) and Haiti (2,269). The sharpest percentage increases in the number of violations were verified in Lebanon (545%), Mozambique (525%), Haiti (490%), Ethiopia (235%) and Ukraine (105%).
(A preemptive note: These are verified cases; so the real numbers will be higher. In both conflicts, verifying every or even most cases is… tough.)
https://www.unicef.org/press-releases/average-least-16-child...
> At least 2,406 children have been killed or injured since the escalation of the war in Ukraine nearly 1,000 days ago, according to the latest available verified reports.
High-end estimate for Gaza (provided by hamas, a well known terrorist group): 64 000. High-end for Mariupol (provided by Uppsala Conflict Data Program): 88 000.
When comparing numbers one should also compare high-end estimates vs. high-end estimates and conservative numbers vs. conservative numbers.
That's total. Now, as the claim is about children, do the proportion of each place under 18.
Half of Gaza are minors, which is quite unusual, and very much not the case in Ukraine.
https://data.unicef.org/how-many/how-many-children-under-18-... (6,609,605 / 37,732,836 )
https://data.unicef.org/how-many/how-many-children-under-18-... (2,415,888 / 5,409,202)
https://www.worldometers.info/world-population/ukraine-popul... "The median age in Ukraine is 41.8 years."
https://www.worldometers.info/world-population/state-of-pale... "The median age in the State of Palestine is 20.1 years."
I will do the proportions as soon as I find more sources on what is the median age of hamas member. Maybe you have some data on that?