Found them in a different section on Findmypast. At least we have a birthdate, although it looks as though Peter Snr has chopped off a few years:
Merchant Navy Seamen:

Last name:MYLCHREEST
First names:Peter
Age:
Day of birth:28
Month of birth:November
Year of birth:1874
Place of birth:Castletown
County/country of birth:Isle of Man
Card type:CR10
Identity certificate no.:350175
Series:BT350
Date range:1918-1921

Last name:MYLCHREEST
First names:Peter Ronald
Age:
Day of birth:15
Month of birth:July
Year of birth:1905
Place of birth:Peel
County/country of birth:Isle of Man
Card type:CR10
Discharge number:
Identity certificate no.:463162
Series:BT350
Date range:1918-1921

FamilySearch has Peter Ronald's bapt date as 27 Aug 1905 German, St Johns.

Sue