Reference: Regulation (EU) No 965/2012 on Air Operations, Annex IV (Part CAT)
Regulation (EU) No 965/2012 Air Operations requires an infant, i.e. person younger than 24 months, to be secured on adult's lap by a supplementary loop belt or other restraint device.
“CAT.OP.MPA.225 Seats, safety belts and restraint systems
(2) The operator shall make provisions for multiple occupancy of aircraft seats that is only allowed on specified seats. The commander shall be satisfied that multiple occupancy does not occur other than by one adult and one infant who is properly secured by a supplementary loop belt or other restraint device.”
The use of 'child restraint seats' on board is currently dealt with at national level, i.e. at discretion of an air operator and the respective national aviation authority that does oversight of the operator's activities. However, EASA had brought up the issue of Child Restraint Devices (CRD) to ICAO in the recent past and ICAO agreed to take up the initiative on a global level.
The approval of a Child Restraint System (CRS) would require either an Authorisation in accordance with ETSO-C100b or an STC. Should any further clarification on certification aspects be required, please contact ETSOA [at] easa [dot] europa [dot] eu