THEY are Hong Kong's youngest maximum security prisoners, three innocent tots, serving time behind bars for the crimes of their mothers. Fei Tsai, eight months, Kei Kei, 19 months, and Ah Ying, 21/2 years, live and play together while locked in the territory's largest and most secure prison for women, Tai Lam Centre. Child murderer Chan Ching-fung, who incinerated her two step-daughters by padlocking them inside her flat and igniting a can of kerosene while they slept, is accommodated a stone's throw away from the tots. ''Of course, she is kept in a place separate from the babies and their mothers,'' said Superintendent Cheung Ming-leung, the Correctional Services Department officer in charge of Tai Lam Centre. Just like their fellow inmates, many of them drug addicts, the children are locked in a section where they sleep in cots at the foot of beds for their mothers. Bars across windows looking out to high fences topped by coils of barbed wire, heavy padlocked doors and a lingering smell of disinfectant contribute to a grim setting, at odds with photographs of smiling babies pasted on scrubbed-white walls. Under the watchful eye of his 28-year-old Causeway Bay mother, who is accused of manslaughter, baby Fei Tsai extended a tiny fist and tried to clench an impossibly large gun-metal grey bar. ''It is better for me to take care of my baby because I can provide maternal love,'' his mother said. The boy's father, also facing a manslaughter trial, is on remand in the Lai Chi Kok Reception Centre. Kei Kei's mother was jailed for being an illegal immigrant with a forged identity card, while Ah Ying's mother, a Vietnamese refugee, is serving a six-month sentence for theft. Prison Rules state ''the child of a female prisoner may be received into prison with its mother and kept during the normal period of lactation . . . and may be supplied with clothing at the public expense''. The same rules stipulate the child must be removed from the prison and committed to the care of relatives or an institution upon reaching the age of three. Volunteering a baby for incarceration in prison is a little-used option in Hong Kong because in most cases a caring relative is available. But there was nobody for Fai Tsai, Kei Kei and Ah Ying.