a 2 year old will need their own seat, meaning where they seat is determined when you buy the ticket.
If you have 2 parents and the child, your only option (in all fairness) is for one parent and child to be in business (or economy if you prefer), and the other parent in economy (or biz, if you so wish).
You could thread the line and have the child in economy, front-most row and the two parents in business class, back-most row, with only a curtain.
But, it most likely wont be allowed for safety reasons, when the seat belt sign goes on for prolonged times during turbulence, landing and take-off.
So, 1 parent in biz and one in economy.
OR, just buy 3 business class/3 economy class seats to amicably solve the problem!