Upvote:2
There are rules for child seats are set by EASA, the European Aviation Safety Agency, which easyJet must comply with.
EASA has a page and linked PDF info regarding approved seats, which must have a marking indicating compliance with one of:
However, EASA gives the same disclaimer that the seat has to be "properly installed on the aircraft seat". Likely, nobody can guarantee that every child seat can fit in every possible aircraft seat and position.