The Court of Appeal has upheld a High Court decision to dismiss care proceedings for three siblings, after one girl said she had been kicked and slapped by her parents.
The High Court judge found that the statutory threshold for making a care order had not been crossed. In the Court of Appeal ruling, Re MA (Care: Threshold)  EWCA Civ 853, Lord Justice Ward said: "This is, as I understand it, the first time this court has had to consider where the dividing line between harm and significant harm is established.
The parents came to Swansea council social services' attention when it was discovered they had subjected a young girl in their care to what a judge described as "shocking treatment." That included regular beatings and threats that a dog would be set upon her.
After the parents' own daughter, known as "M", revealed that she had been kicked and slapped by both her parents, the couple's natural children were placed with foster parents and care proceedings were instigated by the council.
However, Mr Justice Roderick Wood, a High Court judge in Cardiff, dismissed the care proceedings having found that in his view the children had not suffered "significant harm".
Last week the children's court-appointed guardian challenged the decision at the Court of Appeal, where solicitor advocate Graham Jones argued it was "irrational" of Mr Justice Wood not to order that all the children be taken into care. But the court upheld the Mr Justice Roderick Wood's decision.
The hearing related to two homosexual parents, a "wealthy well-educated" father and his wife trapped in a loveless marriage. They conceived three natural children despite only ever having had sex on a single occasion, and thereafter used artificial insemination.
The ruling means the children, who were kept away from their parents pending the ruling, will now be returned to their parents.
Delivering the judgement, Lord Justice Ward, said: "On an unspecified occasion, or occasions, M was slapped on her hand by her parents, she was slapped on her face by her mother, kicked by her father, kicked by her mother, hit on the side of her face by her father, hit to the right-hand side of her face, kicked and pushed by her father.
"It sounds terrible. It could speak of a persistent campaign of abuse causing real suffering.
"But it could equally amount, in fact, to no more than a handful of isolated minor acts of chastisement forgotten as soon as administered.
"Yes, it amounts to ill treatment and therefore to harm.
"Yet, despite intensive outside intervention in this family's life, no-one ever saw a mark on that little girl and the stark fact is that she appeared to be well nourished, well cared for and with close attachments to her parents.
"So was the harm significant? That was for the judge to decide.
"The harm must, in my judgement, be significant enough to justify the intervention of the state and disturb the autonomy of the parents to bring up their children by themselves in the way they choose.
"The judge heard this case for six days. He is highly experienced in this class of case. The nuances would set his antennae reverberating.
"In my judgement he was fully entitled to come to the conclusion that he did. I am inclined to think that I may well have come to the same conclusion myself."
Lady Justice Hallett agreed with Lord Justice Ward's assessment, saying: "Reasonable physical chastisement of children by parents is not yet unlawful in this country.
"Slaps and even kicks vary enormously in their seriousness.
"A kick sounds particularly unpleasant, yet many a parent may have nudged their child's nappied bottom with their foot in gentle play without committing an assault.
"Many a parent will have slapped a child on the hand to make the point that running out into a busy road is a dangerous thing to do. What M alleged therefore was not necessarily indicative of abuse. It will all depend on circumstances."
Greta Thomas, director of NSPCC Cymru/Wales, said: "Currently the law in the UK on hitting is confusing and leaves children of all ages vulnerable to abuse.
"It is a national embarrassment that the UK is one of only five remaining EU countries which have not given children equal protection under the law on assault or committed to doing so.
"There is a danger that the use of physical punishment by parents and carers can escalate and may result in serious harm to a child.
"Changing the law to make physical punishment of children illegal would provide a clear basis for child protection and the promotion of alternative forms of discipline."