We just built one last week and they definitely only let in 1 person with the builder, no exceptions, so if your DH and DD are both building their own lightsaber you can certainly join them and watch, but if only one of them is building and they both will be in there, you will not be able to watch.
Maybe she will change her mind. I was the builder and my hubby was the guest and he thought it was going to be silly, he’s not a huge Star Wars fan like me, but he does really like the Mandalorian, but i digress, after watching me build and doing a video of me, he said it was actually a really cool amazing experience. Our Savi guide was super fabulous in the role and he is very glad we did the experience. We almost decided to build another one, but opted to buy the legacy Mace Windu lightsaber instead since I have always wanted that one too.