Are you thinking of 'breeching'?
I'm not sure you've been told the right thing about newborns and christening.
Dressing in white might simply have been lack of other baby clothes in the days pre-man-made fibres, but wrapping babies in coloured shawls etc was widespread.
Those extra-long 'christening gowns' were put on posh babies not just for christening, but all the time, as the extra cloth was bundled up to soak up the wee - a 19th century baby was basically a widdly sponge.
Little boys wore frocks until they were continent - age depends on individuals - and once they could sprint to the potty unaided they got their first trousers, ie were 'breeched'.
I reckin breeching stopped around the time of world war one, when terry towelling hence terry nappies became more commonplace.