A relative of mine died last year and his solicitor was the executor of his will. As I was the oldest living relative after my deceased relative I was sent a copy of my relatives will and he had left everything to two charities, his house, car, belongings, savings etc. etc. I was not at all upset about him leaving money to the charities and not to me as it was his money and belongings and his right to leave them to anyone he wanted to.
I think if the solicitor is the executor of the will then whatever you have written in the will when you took it out will be distributed to the people you want to have it and your solicitor would ensure that your child with the learning difficulties will be taken care of as he will hold on to the funds to ensure no-one else gets their hands on it. The solicitor has to ensure the money etc. goes where you have requested it to go, but if your step children are executors, you will never know if they will carry out your wishes after you die.