The standard reflexive pronoun form which corresponds to the plural forms they and them is themselves:
I just showed the boys the refrigerator and told them to help themselves.
In current English, they and them are sometimes used in singular contexts, to refer to a person whose gender is unspecified (see also 'He or she' versus 'they'). For example:
If your child is thinking about a gap year, they can get good advice from this website.
In recent years, people have started to use themself to correspond to this singular use of they and them: it’s seen as the logical singular form of themselves. For example:
This is the first step in helping someone to help themself.
This form is not yet accepted by everyone, though, and you should avoid using it in formal written contexts. If you were writing the sentence above, you should say:
This is the first step in helping someone to help themselves.