Leane7
I'm sure they will have a 'terms and conditions' that the customer will have to agree to before booking a forward or exit row, ones that will obviously advise them of who is accepted to sit where (for safety).
Also, when booking a ticket, the site would pick up that there is an infant or wheelchair customer booking so the seats will be blocked for them to choose from. This will be acheived as (I would imagine) there will be a section for saying you've got an infant and/or a section to state whether or not you are a pax requiring special assistance.