This is ok. Mom's don't stay with newborns all the time (she has to eat, stretch her legs, use the litter).
Just be sure not to stress her by hovering a lot (this could cause her to move her litter to places you never expected, in attempts to hide them; or it could cause her to abandon them or harm them). She needs her privacy and sense of safety and security.
As long as she returns to the 'nest' every couple hours for feeding, grooming and warmth, the kitten should be fine. Make sure there's no drafts or access to the little one by any other animal (including children).
It shows you care a lot just for asking - congratulations on your new addition!