My kids are all in their forties now and so I don't have that problem anymore. However, what I would say is, I don't like driving around without a spare wheel and so I have purchased a spacesaver and obviously this takes up valuable space in the boot. When we go off to the caravan (regularly) I find that I have to load up part of the rear seat as the dog takes up one half of it.
If you're not put off by the thought of putting gunge in a punctured tyre (I am) then the boot is of a reasonable size without the spacesaver.
From a personal standpoint, If I were (many years) younger and had a family, I would not buy the IS300h.
I'm just glad that my kids are all grown up and so are the grandchildren, they no longer want to go out with nan and gramps and, this has put me in the marvellous position of being able to own and enjoy my car, and boy do I love it!