It's family stuff. I don't think MK has a formula for helping one declutter one's child's or family heirloom sentimental vaguely sometimes useful stuff that next eldest kid might use or want, etc.
As the only member of my family left, I've ended up with several households full of 'stuff' over the last number of years, including all of the sentimental items. Just too much to really handle for one home. I've been going through working out what is important to me and how I can adapt them to suit my home and lifestyle. I don't want boxes of sentimental things sitting in boxes that never get used.
For example, my father did tapestries, which are very high quality, but were very old fashioned in their ornate gilt frames. Some went to the framers and were reframed in really modern frames and mounted. There is not one person who walks into my house and doesn't comment on them (and are surprised that they are not paintings). I gathered up all the old jewellery pieces from family members (along with a few of my own) and took them to a jeweller and had a really lovely one of a kind pendant made with the gold and stones that I now wear a lot, instead of random old fashioned pieces sitting in a jewellery box that never gets used.
It's ok to repurpose things that were heirlooms so that they can be used and appreciated. There is no joy in heirlooms stuck in a box.
Photos are important, but having twenty boxes of very old photo albums in my house was taking up room and not making me happy. I've been going through the albums, scanning them onto the computer with notes in the file about who/what they are, and then putting the actual photos into file keepers and getting rid of the bulky items. I bought four electronic photoframes, mounted them in my lounge, and every few weeks I update the cards on them with the new scanned photos. That way the photos are actually on display rather than stuck in dusty albums that I'll never look at and taking up space.