And is the only additive that can make two singular subjects in to a plural one.
In addition to, not only X but also Y, etc. will not make the singular subjects in to a plural.
The
boys, in addition to the girls,
are participating in the games.
In the above example you gave, it is not
in addition that makes the subject plural, it is the plural subject
boys that makes it plural.
Even if your sentence were:
1) The
boy, in addition to the girl,
is participating in the games. -> still singular
2) The
boy, in addition to the girls,
is participating in the games. -> still Singular.
Hope this helps.