The children, now aged between seven and 11, live in the same street five yards from a strip of land contaminated with chemical waste from a disused coking plant in Abercwmboi. It is one of many tips in the area. Waste from the same source is known to contain substances such as mercury, asbestos, phenols and ammonia, many of which are cancer-causing.
Vicky Cook's nine-year-old son, Martin Pontin, suffered severe ulcers and nose-bleeds and her four-week-old daughter developed weeping sores which she blames on the tip.
"The children used to play on the field and one day they started suffering from terrible mouth ulcers. They covered two teeth. One boy had so many he couldn't swallow his own saliva. They also suffered heavy successive nose-bleeds." Doctors were baffled. One boy's underarm blisters ate into his armpit and failed to respond to antibiotics.
"We didn't link the site and the illnesses and so we never stopped them playing on the land. There was no fence and the area is very attractive to young children," says Ms Cook. But following publicity surrounding more than 300 cancer cases among former workers at the nearby coking plant, residents became suspicious. The Welsh Development Agency bought the site behind the houses a decade ago, but has only recently put any fencing around it.
Ann Clwyd, MP for Cynon Valley, visited last summer: "I walked over the site with them and saw the skulls of animals and the carcasses of birds, hedgehogs and sheep that were stuck in the soft sticky tar. Children and dogs had played on another part of the site, and the children had fallen ill with sores, blisters and ulcers. There were no signs saying danger and there was no proper fence to keep children and animals away," she said.
Solicitors acting for the children commissioned an independent land survey from a team of specialists at Exeter University. Peter Granger, director of the Earth Resources Centre at the university, says the findings are consistent with the chemicals the parents feared. "There was a mix of heavy metals and organic compounds. It did show contamination above accepted levels," he says. The chemical analysis supports the children's claims and amplifies anxieties that they may be vulnerable to cancers and other life-threatening illnesses.
Under the Occupiers Liability Act owners of property who believe it may contain dangerous elements have a duty of care even for trespassers.
Mike Green, a solicitor with Smith Llewelyn, thinks the children have a strong case. "The situation was always worse during the summer when the hot weather melted the pitch and chemicals would come to the surface."
Vicky Cook also believes that in summer contaminated particles polluted the air causing the ulcers on her baby.
The Welsh Development Agency refused to comment last week but it moved 300 tons of earth from behind the housing to a licensed tip in South Wales.