It's a reasonable question, because "a couple" is singular. However, as it means (about) two, it can be considered as a number and the subject is "girls", which is plural. The English language is in fact not very clear about singular and plural and you can find a lot of bewildering examples.
It should be "a couple of girls are in the park" because here 'couple' refers to two (British English) or a few (American English). For example, a couple of police officers were standing at the door.
In English. there are some things, I find, senseless!!
Or how can you jusify that we deal with"a number of..." as if it is a plural subject while we deal with "the number of..." as a singular subject?? (at least that what I've studied) "a number of" means several what about "the number of" doesn't it have the same sense??
This one is confusing because usually when you have "a [ x ] of [ y ]" we are taught to ignore the "of [ y ]" part and use [ x ] to determine whether it's singular or plural.
A pile of pennies is on the table.
The number of students attending this school is increasing.
But you would have no problem with "Three girls are coming to the party" or