The reason to this caution is simple: the books we read when we were younger are vastly different to what is currently on the market for kids and for adults. So make sure not appear dated in your language, set-up etc. You may think it’s the most amazing thing ever, but honestly, if an agent or editor thinks it’s target audience won’t pick it up, you can forget about it.
Once more this advice comes down to research and becoming au fait with what is currently commerically available on the market.
So, rethink that novel you’re thinking of writing (especially if it’s heavily influenced by a favourite kids’ book / is a homage to one) and if you feel you can’t stand to let it go, then update it dramatically to be more modern, and see where it goes. Or, as one of the excercises mentions: take a scene from your favourite classic novel and rewrite it in a contemporary fashion for a young audience. See if it works. It may very well do so. This could be your chance to give an old classic a modern spin. But do remember to check permissions and copyright etc. as it may become tricky later on.