if you can go home early, maybe you can adjust the feeding and dinner time. For me, i would ask my helper to feed my son (17 months) at 6pm, then it would be finished at around 6:30pm, then, she would go to prepare the dinner and i will play with my son, and we have dinner at about 7pm. Sometimes, my son would come to the dining table and ask for food while we are having dinner, then i would feed him a bit. If my son doesn't go to play after i give him a bit of the food, then, i would ask the helper to eat faster, and i take care of my son first, after she finished (about 10 minutes), i can eat.
I think if she is too old, maybe she has her own style, difficult to teach. My helper is 40 yr old also, but so far she is a responsible and smart helper.
Even though i am very tireyd after work, i think i want to spend more time with my son. Don't give up and you will have your reward one day. Don't let the baby watch too much TV, buy more books and let the helper use it daytime.