The couple, from Pontypridd, have been fostering children for three decades, and say all the worrying goes with the territory.
"The most satisfying thing is seeing children flourish and making them happy again," said Yvonne, who says fostering is good for keeping you young.
"It's hard when a child first arrives because they're heartbroken. They've been taken away from everything they know. And no matter what has happened to them, their mother is still the best thing in the world."
Yvonne and Ken went into fostering after losing their two-year-old son in a road accident. They hoped that offering children in care the chance of a new life could also provide them with a lifeline.
"Since then we've had children with us who started out as short-term placements but ended up staying years," said Yvonne.
The couple, who also have a grown-up daughter and two grandchildren, currently have three foster children including an 11-year old boy and an adopted 12-year-old girl, who is the daughter of the very first child they fostered in 1976.
Their third foster daughter, Kathy, is 16 and was placed with the Masons when she was 11.
When she arrived, she was in a distressed state and had lost interest in everything, including herself. Last month, however, she was honoured at a special ceremony for looked-after children, organised by Rhondda Cynon Taf council, for her voluntary work at the local primary school, and is now studying for her GCSEs.
"There is no way I could have seen Kathy doing her GCSEs when she first arrived with us," said Yvonne. "She could hardly read or spell, and she had probably had about 30 months schooling in four years. I was quite overcome at the awards ceremony - I'm very proud of her."
Yvonne is passionate about giving the children in her care every available life chance. She spends much of her own time on training courses and regards education as crucial. Ken teaches disabled adults in his spare time and is also a part-time caretaker at Coedylan primary school.
"I have never had a problem with any of my children going to school, because they know it's not an option not to go," said Yvonne. "It's just part of the everyday routine of the household.
"I think education is as important for foster children as it is for any child, but children who have been in care do need support and it is important that they are not moved around."
Yvonne says with such a shortage of foster carers across Wales, more people should consider it. "You do need a lot of patience and determination, but the rewards are wonderful."
In RCT alone it is estimated that at least another 35 foster carers are needed. Many children have to be placed with private foster agencies - some as far afield as Bristol - which is costly and disruptive.