Indeed, no-one can be absolutely certain but in the complete absence of any evidence to support the existence of any god(s), let alone the one mentioned in the bible, then we can reasonably assume that there aren't any as a matter of practicality if not strict logic.
It is fine to make a reasonable assumption but, by your own admission, that would make teaching a young impressionable mind that God definitely DOES NOT exist illogical and, in my opinion, unfair to the child.
So if you were teaching a young, impressionable child you would give equal credence to Zeus etc? If not, why not? Not to do so would be, by your own admission, illogical and unfair.
That is what I do. I teach my daughter that there are many myths that many different people believe, and that sometimes these myths tell us great stories, and sometimes there are wonderful celebrations around these myths. My daughter loves stories about Zeus and Jesus and she loves celebrations like Lunar New Year and Diwali. She loves the pagan tradition of putting lights on an evergreen during the darkest days of the year, to remind us that the light will return, and so will the green. And she loves going to light Hanukah candles with friends, and hearing about the lamp oil that lasted so long, and she loves going to an elderly neighbor's house for three kings day, and moving the little king statuettes around close to the little baby Jesus in the manger.
She knows that many people believe that these stories are true, and that I do not believe that they are true, but that sometimes it is fun to tell the stories and enjoy the celebrations.
She also knows that some people think that their religion is true, and that some religions think that some families are bad, and some families are good. This makes her very indignant. She also knows that some people think that they do good things because of their religion, but that I think it is important to do good things all of the time. And if you are not sure if something is good or bad, you should think about how other people feel. Empathy. How will your actions impact on other people? Sometimes the answers are easy. Don't hit. Don't steal. Don't exclude someone from your group because they are different. Sometimes the answers are harder. What if telling the truth hurts someone else's feelings? What if telling the truth gets someone else in trouble? What are your responsibilities, as an empathetic person, and as an ethical person?
I tell my daughter that many years ago, people did not know the answer to many questions. So they made up stories to answer those questions. Different cultures made up different stories. Some people still believe the stories, even when science has provided answers to many of those questions. Most people don't think that they live on a flat plane that is being balanced on the back of a giant turtle anymore. But some people still think that a caucasian-looking god, with a white beard, created the earth and the sun and the whole solar system and the whole galaxy and all of the galaxies in the universe in one week. These are both interesting stories, but she has been to the planetarium, and she knows that neither of these stories are true.
I do not tell my daughter that gods do not exist. I tell my daughter that I do not believe that any gods exist. She can make up her own mind.