A spritz of bleach will make it disappear, but it may come back if there is an underlying problem. Is it just one spot, how big, outside wall?

It's inside the cellar, in the basement underground, on the painted concrete l- it's the only spot
Wondering if I need to leave the house cause I have babies

It's about 5 x 5"

Generally black mold grown on cellulose in drywall not on concrete in cellars. And it occurs due to moisture. I had a cellar in Oklahoma that was cement block and had this issue. I painted the wall with dry lock sealer and the problem was solved but I had to put a dehumidifier in the cellar during the summer month when the humidity was up. I personally would not be concerned unless it was near a leak and near drywall.

However, you need to figure out where the moisture is coming from, fix it, treat the mold and get a dehumidifier in the cellar. Mold is never healthy, but I sincerely doubt this is toxic black mold, it looks too much like mildew. Toxic back mold grown on organic survives where water is present and it has a green haze. Actually was in a house for all of 5 minutes that had it and it was everywhere. The house had to be condemned.

The concentration of spores in the air has to be high. One small patch is not going to hurt you or your kids. Furthermore, if the basement does not share air with the rest of house mold spores are not circulating through the ductwork. You still need to figure out what the cause is.

Tape a piece of aluminum foil over the damp area. Come back the next day and check the foil. If the side you can see is wet, the dampness is condensation. If the side against the wall is wet, the problem is water seeping in from outside.

A dehumidifier is a solution to condensation and HERE is information on how to choose one. If your laundry is in the basement check for water leaks around the washer and check the dryer vent for air leaks.

If the problem is water from outside, the solutions are more involved. HERE is an article that outlines the problems and solutions.