The more active a mother is, the more physically active her child will be, suggests a UK study of 500 mums and four-year-olds.
But many mothers' exercise levels fell way below recommended levels, it said.
Researchers from Cambridge and Southampton universities used heart-rate monitors to measure activity levels over seven days.
The study, published in Pediatrics, said policies to improve children's health should be aimed at mothers.
Children are not "just naturally active", it concluded, and parents have an important role to play in developing healthy exercise habits early on in life.
As part of the study, 554 four-year-olds and their mothers from Southampton wore a lightweight combined heart-rate monitor and accelerometer on their chests, for up to seven days.
Participants wore it continuously, including while sleeping and doing water-based activities.
Kathryn Hesketh, now a research associate at the Institute of Child Health at University College London, co-led the study and said the data from mothers and children showed a direct, positive association between physical activity in children and their mothers.
"The more activity a mother did, the more active her child. Although it is not possible to tell from this study whether active children were making their mothers run around after them, it is likely that activity in one of the pair influences activity in the other."
She said that for every minute of moderate-to-vigorous activity a mother engaged in, her child was more likely to engage in 10% more of the same level of activity.
For every minute the mother was sedentary, children were 0.18 minutes more sedentary, so one hour of sedentary time in mums would result in 10.8 minutes in children.
These small differences may seem trivial but over the course of a month or a year they could be significant, she added.
Factors which influenced a mother's activity levels included whether she worked or not and whether the child had brothers or sisters.
Co-study author Dr Esther van Sluijs, from the Centre for Diet and Activity Research at the University of Cambridge, said the link between a mother and child's activity levels was stronger for mothers who left school at 16, compared to those who left at 18.
Changes in motherhood
The study recognised that once women become mothers their activity levels fell and often failed to return to previous levels.
This lack of activity could then influence their young children, it said.
Miss Hesketh said: "There are many competing priorities for new parents and making time to be active may not always be top of the list. However, small increases in maternal activity levels may lead to benefits for mothers and children."
Simply walking and moving more each day can be all it takes to create these benefits.
Dr Ann Hoskins, director of children, young people and families at Public Health England, said it was committed to increasing physical activity in families and children to improve overall health.
"Active play is an important way to develop coordination and motor function skills in the pre-school early years and there are lots of activities like parent and toddler swims, buggy fit and baby gym which provide opportunities for mums to socialise, be active and support their child's development."
Their Change4Life campaign encourages families to eat well, move more and live longer. For children, that means being active for an hour a day and two and a half hours a week for adults.