Changing passenger on flight?

not baby related but I have a question... I'm due to go away next week with my partner but due to me booking last minute, his flight was full and I've had to book somewhere else with my 1 year old and being pregnant. One of his family member s on that flight, now isn't going. so I was wondering if anyone thinks it will be possible to change the name to mine but also add my daughter as a lap seat so he can help me out. it will be a very long flight. does anyone have any experience or think this will be possible. I don't want to fly alone 😭 thanks in advance