While Hamas may or may not be exaggerating, news agencies like the BBC and organizations like the UN also report on casualties affecting their personnel and affiliates, as well as those people's families and it doesn't look all that great there either.
UN is at 101 KIA from its staff so far.
From the hustle and bustle of New York City to the edge of the Karura Forest in Kenya’s capital, Nairobi, offices across the global UN system paid tribute on Monday to the 101 staff killed so far during the war raging in Gaza – the largest loss during a conflict in the Organization’s 78-year history.
UN has 10K staff in Gaza so that's a 1% death rate.
The number of area staff range from nearly 3,000 in the Lebanon Field Office to over 10,000 in the Gaza Field Office with the other three Field Offices falling in between those numbers.
So 10K civilian dead out of 2M is not unbelievable. The UN staff may be more exposed, staying to help near combat areas, than civilians in "safe zones". But UN working staff should also be in clearly marked vehicles and buildings.
These stats falls very much in the general ballpark figure where Hamas numbers aren't automatically suspiciously high, no matter the wishful thinking to claim it so at least in the absence of additional information.
Still, I would very much also reason about these numbers on the basis of Arno's answer - take them with caution and keep in mind who issued them.
Quoting the Guardian's article cited in a comment, there is probably one area of contention: who gets counted as civilians vs Hamas in the casualties. Even then they cautioned against too much doubt.
Shakir said a grey area was differentiating combatants from civilians among the dead, but the large proportion of women and children killed was indicative of high civilian casualties.
One last thing to keep in mind when reasoning about casualties wrt children: Gaza has a very unusual age pyramid going on and nearly half of the population is under 18. So their proportions in the overall casualties are going to be different from what you'd expect from a war like that in Ukraine which has proportionally much fewer children. Make of that what you will, but it is a relevant data point.
Nearly half (47.3%) are under 18.