What service do the boys provide at the out houses?

Probably several things combined:

• they keep it clean so it is more attractive than the outdoors solution
• they give people using the stall a sense of security (=nobody will disturb them)
• when it's the only option around, people have no choice but to pay them, and they just take advantage of it

In this article proposing a possible alternative to Mumbai's lack of sanitation, it explains that without a sewage system, people have a choice of defecating in the open, in a government toilet if there is one available, or in a private pay-per-use toilet. Urination is free for men using urinals or nature. For women, urination is more problematic. They can squat in the open, or they can use the paid toilets which are really designed for defecation.

Salim's job in the film was to operate a pay-toilet.

Here's some possible background on the situation you see:

In Mumbai, a Campaign Against Restroom Injustice

...Almost always, a male attendant oversees these toilets, collecting fees. Petty corruption is rampant in India, and public toilets are no exception: Men must pay to use a toilet but can use urinals free (based on the premise that urinals, usually just a wall and a drainage trench, do not need water). But women are regularly charged to urinate, despite regulations saying they should not be.

...The toilet fees might be considered nominal, ranging from 2 to 5 rupees (about 4 to 9 cents). Yet in India, the poverty line is so low that the government recently defined the urban poor as those living on less than 29 rupees a day.

It seems unlikely that men use these toilets very often anyhow. But then it also seems unlikely that children would be guarding them rather than grown men. And they probably don't get cleaned very often. Has anyone read the book Q&A that the movie is based on?

They are men's-room attendants.

Those outhouses are pay-toilet; each person using one must pay, probably 1 rupee (about one-and-a-half US cents). In the West, where labor is expensive, a pay-toilet has some automated mechanism to collect the fare, but in the poor countries, there is a human attendant with the same job.

I have seen this in Southeast Asia, where there is a strong cultural emphasis on both cleanliness and privacy, but not (except for this movie) in South Asia, where (completely!) public elimination is so common that it is a health hazard and the government struggles to get the population to use the free toilets provided.