If you say "I don't mind if you come late" - It means either I don't mind if you are late (as in not on time) OR it can mean I don't mind if you get here late (as in, late in the day like at night)
If you say "I don't mind if you come in late" - It implies coming into somewhere, like coming home late, or coming in to a shop late.
For the context where you are using it here, it is correct to say come in because you are talking about coming home.