Joanne Curren, 44, and three-year-old son Noah are set to make the two week trip to the USA in July to visit her parents in California.
She booked the £1,600 British Airways return flights through Thomas Cook, who charged the pair £25-£35 extra per seat on each leg of the journey to sit together.
The mother-of-one claims her son will not remain in his seat if he is not next to her, and could disrupt other passengers.
Ms Curren, who works in a school kitchen, said she and her son travelled to America last year with BA without paying extra or pre-booking.
She added: 'He will have just turned four when we go - he won't stay in his seat if I'm not next to him. What I really want to know is what will Thomas Cook do about the safety aspect.
'Surely the plane won't be able to take off if he is getting out of his seat? That would mean they would get fined if it has to stay on the runway.
'What if there is turbulence over the Atlantic? How will that be dealt with, will the person who is sitting next to him have to fit his mask in the event of an emergency?
'Are they going to be happy sitting next to a four-year-old for ten hours?
'He is not confident enough to sit on his own so he won't stay in his seat, he'll be running up and down the aisle. He'll probably be kicking the seat in front as well.
She said: 'I feel like they are holding people to ransom so pay extra just to sit next to family members'
'Disabled people automatically get seated next to carers, they don't have to pay extra - young children should be in the same category.
'It would be fine if he was seven or eight, but he'll only be four.'
In reply to her concerns, a Thomas Cook representative said on Facebook that the company do their best to sit families together but cannot guarantee it.
The message read: 'We always try to sit families together where possible.
'However, due to other customers pre-booking seats, we aren't able to guarantee that there will be seats available together at check-in.
'The only way we can ensure you're sat together would be by pre-booking your seats.
'Once the plane is booked up and there's not enough room to seat all the family members together, the only option would be for cabin crew to politely ask people who've booked their seats to move.
'However, passengers who have pre-booked their seats can refuse. The best and only way to ensure you are all sat together is pre-book your seats so that this doesn't happen.'