I've just found the topic where Drladybird posted the multi-veg muffin link, and it's here:
www.mumsnet.com/Talk/food_and_recipes/1827270-Savoury-muffin-recipes
There are other suggestions in that thread.
There's also a vegetarian Christmas thread, which might give you more ideas:
www.mumsnet.com/Talk/food_and_recipes/1926911-vegetarian-christmas-the-ultimate-challenge
I'm of the view that what's important about the 5-a-day thing is the nutrients from veg'n'fruit, plus the fibre that's important for the health of the gut. I think it's really a modern-day European style of cooking that presents each vegetable or fruit as separate, whereas in other culinary cultures most of the constituents of the meal are cooked together. That style of cooking isn't disguising vegetables or fruit, it's just a different kind of cooking style.
Also, there's the issue of young children's taste being very different from adult's. Tomatoes are a good example: To me, the flesh of a tomato is delicious; the feel of the seeds in my mouth is disgusting; and the taste of tomato skins is like what cat's piss smells like. And any fruits that are acidic irritate my stomach.