potentially dangerous
If you consider the safest position in an emergency situation is the brace position, ask yourself if you would be able to adopt that position with an infant on your lap. This poses a hazard to both the child and the parent.
Then imagine, if a sudden decelleration did take place, the forces on the infant as the parent is violently thrown forward, not to mention the risk of crushing between adult and the seat in front.
The issue of the seats is problematic, but if the infant is booked in advance, then the airline will have to make an accommodation. The seats may have to be stored at the airports, and put on the flight when the seat is booked.