Raja Ibrer Faisel, 19, of Birmingham, admitted causing death by dangerous driving. He was remanded in custody and will be sentenced later this month.
Sarah Hunt, 28, her son Kieran, nine, and his brother were using a pedestrian crossing when Faisel crashed into them.
He then tried to leave the crash scene, on the A45 in Hay Mills, but was stopped by members of the public.
Mrs Hunt, who was 30-weeks pregnant, was pronounced dead at the scene of the crash on 3 October.
Kieran died in hospital a few hours later and his seven-year-old brother suffered multiple injuries.
Faisel was originally charged with Mrs Hunt's manslaughter but the prosecution accepted his not guilty plea.
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