As a new mum could you help me clear some queries: the whole Pak thing is getting overwhelming for me
a) How do I make the mattress and sheets Pak for an infant who is four months old and is exclusively formula fed
b) Are wipes sufficient in making an infant clean after diaper changes

a) Sheets which have been soiled with urine will be made paak if washed with qalil water twice or with kurr water twice. The ruling on kurr water being used twice is based on obligatory precaution hence you can follow another marja such as Ayatullah Wahid Khurasani who says washing with kurr water once will suffice.
b) As for using a wet wipes, it does not make anything Pak. Since the child does not have to pray namaz or do anything that requires taharat, there is no problem if the area is not made paak