Permission to travel from other parent if within US?

My daughter and I live in MA, BD lives in WA. I have a business trip coming up in Sep in Seattle and was planning to take my daughter with me so she can see him and his family and so I don't have to leave her for 10 days. She's 4mo and we don't have any official documentation stating custody since we wanted to keep things out of court (probably no the best idea, but it's working so far). I did make him sign a letter when I was pregnant giving me permission to move to MA and raise her there, but it's not notarized or anything.

Anyways, I was wondering if I'd still need a certified letter of consent from him if I'm traveling within the US? The only info I can find is for international travel.

Noooo you can travel anywhere you want with your child. Also you didn't need his permission to move...you are a big girl.

OK good, thanks. I don't want to get to the airport and have them turn us away!!

I had read a lot about it and saw that it's against the law to move a child out of state without the other parent's permission. She wasn't born yet, but I did it just to cover msyelf in case he miraculously started giving a smurf and tried to take me to court to try and prevent me from moving in the first place. Doubt he would have won if he had tried, but waiting for the decision would have caused her to be born in WA and therefore caused problems for me moving with her after she was born since WA would have been her state of residence.

You won't need anything. Keep in mind you have nothing in writing, so what is stopping him from keeping her when you allow her to visit?

Part of me wants to lie and tell him that I need to establish custody to travel with her for that very reason. But he has no means to support her and can hardly handle the responsibility of being a long-distance father, can't imagine he'd want to be a real one! It will mainly be his mom and sister that she'll spend time with.