It is a really bad idea to try to keep this secret. Here are my experiences for what they're worth:
My sister died at age 50 of a brain tumour. She had never felt loved by my parents and idolised my Dad to the extent that she even planned her funeral to please him. It didn't as he is ashamed of his heathen spawn.
Anyway 5 years after she died he revealed to the entire family that she wasnt his child, my mum had had an affair with his brother and he had also had an affair. They stayed together and had 4 more children.
No wonder she never felt loved. Now none of us feel that our childhoods were real. Mum died shortly after.
Also
My friend has 2 children with different fathers, the father of the youngest has always treated both as his own, even though they split up when the youngest was only 2.
She never told the eldest, which was silly because they are racially very different. The eldest worked it out when he was 11 and his brother was 8 and it devastated them both. The eldest is still reeling from it and considers his whole life to have been a lie. He has anger issues and is no longer in mainstream school.
You dont want to wait much longer. Children under 6 are very amenable to the truth, whatever that may be.