Lisa applied for Jack to start at Baddow Infants in November last year so that he could be at the same school in Baddow as siblings Henry, 8, and Gabrielle, 10.
The family, who moved from Baddow to Galleywood four years ago, have since been told that Jack can't have a place.
Lisa believes she is locked in a system that she says fails to recognise her children's welfare.
She said: "We applied in the usual way. We heard back in April that Jack hadn't got a place because we now live in Galleywood and the school has been oversubscribed.
"He has been on a waiting list since then but, if other children move into the area, then he drops down the list.
"Even if people have missed the deadline, they still could get a place before him.
"We appealed against the decision and told the council that I can't physically manage to collect the children from two schools at 3.10pm.
"They said I would have to look at childcare or rely on friends, but childcare would cost us around £100 a week, because after-school clubs are charged per hour per child."
Lisa, who works as a health care assistant at St John's Hospital, has also considered moving her older children to Galleywood Juniors, but was unsure whether they would be given places.
She added: "Why would any parent want to move two happy children to a new school, particularly given Gabrielle is in her last year and she doesn't need the disruption? Essex Council Council have failed my children.
"It makes no sense to separate sibblings. I am a good parent but I can't be in two places at once."
An Essex County Council spokesman said it was Lisa's decision to keep her oldest child at Baddow Hall Infant School after moving home, instead of transferring to the local school in Galleywood.
"She applied to send her second child to Baddow Hall and at that time was able to gain a place from outside the area because of lower numbers of local Baddow pupils.
"However, this year a place could not be offered for her third child as there are more local children needing a place at Baddow Hall and her application does not rank high enough according to the published admissions criteria for the school.
"Pupil numbers fluctuate from year to year and parents are advised that places cannot be guaranteed for out-of-area younger siblings.
"This is an unfortunate situation, but it is ultimately the parent's responsibility to ensure their children attend school each day."