A Bendigo mother of five was having sex with a 14-year-old boy when his mate walked in on them, a court has heard.

The mother, who was 28 at the time, later told police the boy had been all over her "like a typical teenager".

The boy boasted to his mate how the sex with the woman had been "the best".

A Melbourne judge on Tuesday placed the woman, who gave birth to her sixth child last year, on a two-year community corrections order and ordered her to perform 200 hours of unpaid community work.

She had pleaded guilty to one count of sexual penetration of a child under 16 and was placed on the sex offenders' register for 15 years.