When Tyler was that age, I usually mixed the yolk with a tiny bit of milk and scrambled it. The consistency was still a lot like a hard boiled egg yolk and he had a hard time picking it up (too crumbly), so I'd mash it up in his puree or cereal. I'd do the same with hard boiled egg yolk. Tyler's all time favorite was egg yolk french toast, though. I'd dip his bread in the yolk and fry it until it was all cooked on both sides, then I'd tear it into bits. For the longest time, I dipped the outsides of all of his sandwiches in egg yolk. It was super easy for grilled cheese, since you have to fry it for a bit anyway.