1. We can massage old people's backs.
2. We can help them use a computer.
3. We can visit their houses and clean rooms.
4. We can take them to a bathroom in wheelchairs.
5. We can help them wash their faces.
6. We can help them take a bath.
7. We can prepare towels and soap for them and help them take a bath.
8. We can help them walk to a bathroom and wash their bodies.
(Are the expressions above all grammatical? Would you check errors? Thank you.)
All are grammatical, yes.
The only thing I would change is to use "the" in front of "bathroom" instead of "a."
7 and 8 seem repetitive. You said you can help them take a bath in 7 and help them wash their bodies in 8. That's really the same thing.
All of the expressions you provided are grammatically correct. However, there are a few minor suggestions to improve the language use:
1. We can massage the backs of old people.
2. We can assist them in using a computer.
3. We can visit their houses and clean their rooms.
4. We can transport them to the bathroom in wheelchairs.
5. We can assist them in washing their faces.
6. We can help them with taking a bath.
7. We can prepare towels and soap for them and assist them in taking a bath.
8. We can help them walk to the bathroom and wash their bodies.
Note that these suggestions are optional and the original expressions are still correct.