I find with the picky eaters keeping it basic and building on what they will eat best, it is a slow process requiring patience and trial and error, whilst keeping the experience positive for both the parents and children, as you know.
Thankfully, there are so much GF products on the market that are great substitutes. I would recommend GF pastas as a start (penne is popular with this age group) and is versatile with what can be added to it or kept plan and adding different fruits veggies to their plate. There are a lot of great healthy recipes on the below link, including cookies, snacks and main meals.
Stand out recipes on there were:
Greek meatball bites (turkey mince)
One pot taco spaghetti
Gluten-Free Kids Recipes | BeyondCeliac.org
I would also offer them rice crackers, gf cereals etc to ensure they are receiving enough grain.