What sorts of things do you do with them? What games does DS play on his own? How have you tried to insert yourself into them?
It can take a while to connect with such a PT role, but with a little more information I'm sure people could offer some ideas.
I had a PT job with a 2 and 4 year old. I won them over with a number of activities - mini hikes, juggling performances, long narrated storytelling sessions, making art out of flower petals and twigs, just generally convincing them I was an interesting person. If you tell us more about the 4 year old we could suggest some things to try.