I've been starting to think about starting solids for my twins, as they will be 6 months old in early August. But there's no way they would be ready for a high chair yet. They can hold their heads up, but are still very wobbly! I prop them up with cushions but they are very slumped and don't sit straight. Is this normal, should I be worried?? They were born a month early so perhaps I am expecting too much...