She thinks everything isn’t safe .
Yea bm thought my ss was gonna drown on our watch bc she had a dream. It's still not a good excuse. If you keep them sheltered from the world they are going to grow up not knowing how to survive on their own. Might be a conversion that your husband has to have w her. Bottom line kids are kids, they get hurt and they heal. They need to explore and learn outside the confines of the home or they will only have limited knowledge.
It could be trauma on her end in the past. My husband (BD) refuses out of country or cruise ship traveling till our kid is older, because husband was always left on his own by his mom. We made an agreement that he can get a passport when he's 10. Always have bio parents discuss sensitive topics. It just leads to less headaches if it doesn't work out.
BM can't legally say no if it's on your time. Just make sure you tell her a head of time and bring the parenting plan with. The only time you might need a signed letter from BM giving permission is if you fly or leave the country.
What is the moms reason to say no. If it's your time with the SD and she doesn't have a valid reason then she shouldn't say no