Hi,
My 84-year-old parents live with me. My father has mild dementia and uses a walker.
My mother loves to work in the yard, but is uncomfortable leaving Dad inside alone for too long.
So I’d like to try and find them a walkie talkie system that works well inside to outside and visa versa.
The simpler and more hands-free the better.
Any advice/suggestions would be much appreciated. 
Susan
The only thing hands free is a baby monitor.
Everything else you are going to need a PTT type system.
Many of the Motorola Talkabout (FRS/GMRS) handheld radios have voice activation (VOX) with volume sensitivity adjustment for transmission as an option instead of just push to talk (PTT).
I’m not sure how well this would really suit their needs, but it seems like an option, at least.
The VOX hands-free feature on walkie talkies isn’t going to work great for your needs. Unless the radio were near their mouth or they are wearing a headset you’d have to have the sensitivity very high to properly detect a voice, and then it would probably cause a lot of unintentional transmits.
I would suggest you take a look at the Motorola Talkabout MD207R. This is a mid-range model, but it has a very large push-to-talk button which should make it pretty easy to use. On this radio you also have to go into the menu to change channels, so once you get it setup it is not likely that they would press the “wrong button” and change a setting that would make them unable to communicate.
I agree with the vox here. Not a good option. Another good choice is a little more expensive but they work real good and have a very large PTT button on the top part of the radio. They come with a belt clip which could be attached to a walker with a tie wrap of some sort. They can be used on the VHF M.U.R.S. frequencies with not license.
http://www.buytwowayradios.com/products/motorola/motorola-cls-1110.aspx