For seeing flowers, the autumn leaves, etc., there are some other places you can go other than Hokkaido (of course, it's beautiful in Hokkaido).
For the winter, with lovely snow and everything white, I believe Hokkaido is the best choice.
We went to Hokkaido in the early winter, and that's one of the most memorable trips we've ever been to. Lots of good memories, romantic, nice snowy scenery, beautiful Hakodate night view, etc. and etc.
And your girl appears not so small any more and if you feel comfortable, it's good to bring her to see, to touch and to feel the snow. If she likes animals, visiting Asahiyama Zoo will be more special in the winter than any other seasons.