Similar story with our DD. I am so so sorry for your daughter and you, OP. This is just the very beginning of everything, and you probably feel overwhelmed and alone. It is an awful thing to go through as a family, but it is so good your daughter felt able to talk to you. Many children never do and suffer because of it.
Apart from going to the police, and please do this asap, your next step will be therapy for DD and you. You also need to remove all contact with her abuser straight away. Talk to the police about this and not to your ex. Let them advise you how to do this.
Please remember, abuse between 6 year-olds is almost always because the abuser has been sexually abused. Sexual knowledge at that age is just about nil, and that the suspected abuse of the boy will be for your ex and his partner to deal with. Not you, not your daughter. Your focus is your daughter, regardless of what anyone says.
Also, document absolutely everything - every conversation, every letter, reaction, then, time date and email it straight away to yourself so you have a date-stamped electronic trail. This is particularly important if people deny being told or problems occur in the future.
Lastly, but also very unpleasantly equally important, you may need to get your daughter tested for STDs. The reason for this is that children who sexually abuse have most likely been sexually abused in some way and may have contracted an STD from an adult. This will be discussed by a doctor with you. Just so you are forewarned.
All the very best, OP. I am here if you need to DM