To turn a specific baby instance into a dog, you need to pick this baby instance first. There are many ways you can do the picking - by instance variable, position, IID, UID etc.
In your case you can add a spawn counter global variable. And also a boolean instance variable "convertToDog" to baby sprite.
When a new baby is spawned you increment the counter and if (counter%7)=0, then you set convertToDog=true for this baby instance. And then you will be able to pick it using "Is boolean set" event and replace with a dog.