Invest in a high powered blender and make fruit shakes for breakfast. That way they get the whole fruit, not just the juice. Then you can also sneak in some great foods like walnuts, and kale!
Since you have not shared where your kids fit on a growth chart, its hard for us to give good advice. I know a family that finally gave into a G-tube for their non eating son. Though it was difficult, he is doing much better since receiving better nutrition through a tube. I'm guessing your case is not this extreme. On the other end of the non eating spectrum, you can take the approach of just letting them fast until they finally are hungry enough to eat the healthy stuff. (we all know we cannot force our kids to eat). A normal child will not allow themselves to starve. But this is easier said than done, and not the right approach for all children.
I have 2 non -eaters and at times have given into crackers and cookies for fear they would starve. But then I took my emotions out of the equation, realized I did not have extreme cases of non eating, off the growth chart children. One of mine is 3rd percentile and one is 35th percentile for weight. I work harder on my little one to eat. I let my bigger chid fast before I give into junk.
I came to this conclusion when I realized that animal protein, animal fat, and sugar actually do damage to the delicate endothelium layer of your blood vessels, through inflammation. On the other hand, fasting is actually an excellent opportunity for the body to heal and repair itself. Obviously you don't want to starve your child, but going several hours without food is not bad for them. Cookies are bad for them. Not just in theory, not just because they can cause weight gain or tooth decay. No, butter sugar and while flower wreak havoc on your vascular health. And thats what Americans die of by the droves as we age- disease of the vascular system.
My advice is get over the idea that your kids need meer calories or they will somehow starve. Our bodies need nutrients; vitamins and minerals are vital to every cellular and metabolic function in our bodies. For this you need plant based foods in the most whole and non processed form.
Only you know your child and his nuances and medical condition. It may not be the right choice for you to let your child get hungry enough until they willingly eat healthy. But if you think they are normal enough to try this approach, I would.
---One mom suggested you not make smoothies. But so long as you know how to make them healthy, there is probably no better food for your son who likely has some sensory issues. Its a great way to get them to eat the whole fruit.