Professional grade items are usually a great start. Not normally advertised, ugly as hell, but powerful, reliable and have a spare part/repair market. A professional vacuum will be expensive but you will be able to give it to your children.

One thing that you can look for is repair manuals, schematics, and spare parts. If those are easy to find, the product may have been made to be repairable.

I try to find as many forums as I can for people who are more likely to know about the product, like enthusiast forums for things like headphones, and professional forums for things like washing machines. I try to get a feel for what parts of a product fail most often, then try to find products where people have specifically reported those things holding strong. There's probably more I could do, but even just that has led me to finding products that have lasted far longer than buying the cheap stuff on amazon had gotten me.