A.P.
Let me just tell ya.... I totally feel your pain. My two oldest children (who are now 10 and 8) were very picky eaters when they were toddlers and ate like birds. It really stressed me out. So with the advice and ideas of mentoring mom's, here's what I did.
Breakfast: LET THEM PICK whatever type of breakfast food they want and give them juice or milk to drink. (No snacks-unless if they seemed like they were going through a growth spurt - otherwise, the kitchen was closed and I am not the maid)
Lunch: I PICK WHAT THEY EAT and yet I pick something I know they like. (Offer a healthy enjoyable snack at a reasonable time between lunch and dinner so that it's NOT too close to just finishing lunch and yet close enough to dinner that you have a content but hungry child when you get to the table for dinner)
Dinner: I BUILD DINNER AROUND WHAT MY HUSBAND AND I LIKE and not around the kids. The rule at our table is even if you don't like what you are served... tough toenails. No comments about how much you don't like the meal or something on your plate - that is offensive to the person (whether it be someone hosting a meal for your family, a neighbor, mom, dad - this is a good time to train the child manners and self control.) Another rule we have is you must take a least 1 bite of everything on your plate. This is a good opportunity to teach your child obedience. I am not worried about quantity of food eaten at this point in the day because I know that my child has eaten a good breakfast and a good lunch. So, dinnertime is used as training in expanding tastes and obeying. NEVER FORCE eatting. Ultimately, your child needs to make the choice to eat or not to eat. I strongly recommend a CONSISTENT CONSEQUENCE if your child does not do what you are requiring.
I always serve milk with dinner and maybe you can use the Pediasure (use according to the instructions and doctor's guidance first and foremost)so that if dinner is not successful, at least you know there is something on their tummy when they go to bed and it puts your mind at ease.
Take all of this with a grain of salt and do what fits your family best.
We are truly reaping the benefits of early years of training. Mealtimes are sooooo much fun now.
Sincerely
A. P