Good question. But first off, good on you for encouraging your granddaughter's interest in history and making time to take her to these sites.
Some Holocaust survivors were so traumatized by their wartime experience they have a rabid aversion to guns as they associate them with the violence perpetrated against them. (Other survivors aren't about to ever give up the tools to defend themselves.) None of us can put ourselves in their shoes to begin to imagine their worldview.
It's very likely that many major donors, board members, and supporters of the Dallas museum are very liberal. Security concerns these days at any place that has a relationship with anything Jewish are necessarily very hightened. Perhaps their security consultants advised that unlike a synagogue or church that allows congregants to carry, and considering the congregants are known to regulars there, having random strangers wander into a place like a Holocaust Museum carrying firearms would allow a person with evil intent unfettered access. A good defense plan is layered so they may see metal detectors as desirable layer, and again, circumstances are different in this particular type of museum than in say an art museum or place of worship.
More puzzling is how many in the Jewish community are opposed to our freedom to own firearms. A Holocaust Museum necessarily includes a tribute to the Warsaw Ghetto resistance where a small group of untrained, poorly armed, sick and starving Jewish fighters held off the most powerful army in the world longer than did the entire country of France. (Worse is how a group like PolitiFact claims that if Jews had weapons that could have prevented the Holocaust which totally denies the success of that same resistance.)
The message of a Holocaust Museum isn't just a Jewish story...it's a lesson about what can happen anywhere when racism, bigotry, and hatred spiral out of control and no one does anything to stop it. The nature of donor driven funding has some of these museums taking on a more general human rights message that tends to stray from the original. The other reason for a change in the way Holocaust museums present their message is that less observant Jews have allowed their politics to replace their Judaism.
Neither of these speculative explanations are satisfactory, IMHO.
Disclaimer: I was a volunteer docent at Holocaust Museum Houston for about eight years and my parents were Holocaust survivors, I try to not let the passion of my intimacy with the topic cloud my perspective. Our museum was also posted and I've no comment to make here about how I prepared for my tours nor what I saw as my responsibility to the safety of groups I led through the museum.