Is this a good idea for a novel?

If you just want to write it for fun, fine. If you have ambitions to get published, then I can tell you that no publisher is going to look at a Percy Jackson clone like this any time soon. However dissimilar your book is to Percy Jackson, publishers are looking for the *next* big thing, not the current one. By the time you've written your book and are ready to submit it to publishers in a year or two's time, they will be 100 per cent over the whole demigods thing and will not touch a story like this with a bargepole.