Up to now, no-one appears to have quoted the Conditions of Carriage correctly. The key Condition is "An adult with two or more infants aged six months or less cannot be accepted for travel."
This should have been caught at check-in
EasyJet Reg's do not allow child car seats for infants under 6 months.
Anyway the seat should have shoulder, lap & crotch straps and be forward facing.
Their requirement is in excess of an EU approved child car seat Cat 0 so I doubt many would be permissable
Business sense suggests EasyJet should provide an approved seat at booking/check-in for a nominal fee (if any) since they will charge for the extra a/c seat reqd (with tax)
Rules may be rules, and I don't criticise the crew, but silly rules can be changed, as BA and Ryanair have found to their cost. (crosses and wheelchairs) Single parents with 2+ kids, no car or immediate family cannot fly?