Let

[Jellybaby]

¿Cómo se dice..."

"If we let her come..."

"Si dejaramos que viniera..." o es que venga?

[Julvenzor]

Two options:

Si dejamos que venga
Si dejáramos/dejásemos que viniera/viniese.

[poli]

Would it be incorrect to say: si la dejamos venir

[AngelicaDeAlquezar]

Although it's often used, and it wouldn't sound incorrect in colloquial speech, the most proper constructions follow the rule of using the infinitive when both verbs refer to the same subject and the subjunctive when there are different subjects.

Here and here are some discussions and examples about it.

[Julvenzor]

Would it be incorrect to say: si la dejamos venir

No, it's correct. I said "two options" metaphorically. Also: si la dejáramos/dejásemos venir.