So I've been wondering, if a genasi has kids with a race other then genasi what will the child be? and if a genasi has a kid with someone of the same race is it guaranteed that it will be a genasi as well?

The description of Genasi says that Genasi are born to:

Mortals that mate with Genies

Mortals that hang around elemental areas

If both their parents are Genasi

and sometimes (not always ) if one or more of their ancestors are Genasi.

So basically, most of the time the kid should be like the mortal parent, but sometimes it is genasai.

Using the phenotyping of Gregor Mendle (a knowledge cleric & western monk) assuming no dominant trait factors, you can expect to see trait expression in any generation equal to the percentage of that trait in earlier generations. (1 out of 4 grandparents a genie? 25 % chance of a genasi grandchild.)

OR whatever the DM says.

If two genasi have kids, the child will be a genasi with a 50-50 chance of being one parent or the other's element.

until they introduce playable Quasi and Paraelemental Genasi, then there's more of a chance of a combination.