And info on car seats here:
No more than two infants per accompanying person are allowed. Infants under two weeks of age will not be accepted for travel.
Children aged two years or over must occupy their own seat and pay the same fares as adults.
It is the accompanying person's responsibility to ensure that the minor is adequately secured into the aircraft seat. To this end easyJet will accept a car seat supplied by the accompanying person for any child over six months for whom an airline seat has been purchased, provided that the car seat has a rigid metal or plastic frame, is no wider than 42cm, includes lap, shoulder and crotch straps, and is upright and forward facing. The required restraint device for the age of the infant is detailed below:
Between 2 weeks - less than 6 months: infant/extension seat/safety belt.
Between 6 months to less than 2 years: infant/extension seat or suitable car-type safety seat.
Between 2 years to less than 3 years: any suitable car-type safety seat or seat/safety belt
3 years or more: seat/safety belt.
If an adult is travelling with more than one infant under the age of two years, but no less than six months, one infant may sit on the accompanying adult's lap and the other infant(s) must occupy separate seats and be seated in a car seat in accordance with the above table.
An adult with two or more infants aged six months or less cannot be accepted for travel.
A child between the ages of two and three years may be seated in a car seat in accordance with the above table.
here