Do you need to tell them? Is this new relationship going to last?
Divorced parents often date a number of people before deciding to settle down with one.
Children don't need to know everything about their parents' dating lives.
Whenever a relationship 'gets serious,' it is a great idea for you and your ex to tell the children together.
Being mature and united in front of the children will help them accept the new situation more calmly and with a feeling of security.