If a child does not have an IEP but was told the school is drafting one, can a child with behavior issues be denied to go on a field trip with the rest of his kindergarten class?

That's going to depend on many factors.  If he/she has a diagnosis of a disability and the school knows about it, suspects it, or should have known, then this could be discrimination.   A child with a disability cannot be excluded from activities in which his/her peers are participating.  They may not have any supports in place (I'm assuming the child also does not have a 504 Plan) yet, but options could be explored:  Could the parent go with the child on the field trip?  A school staff member?  (However, this would depend on the behavior and whether the school has someone trained to deal with the specific behavioral issues.)

It is also going to depend on the severity of the behavioral issues - is it something that could cause harm to the child or others on the trip if he/she participates?  Not necessarily a deal breaker if supports could be put in place, but something that would be relevant.

If there is no data or evaluation yet that shows a disability, it will be harder to argue inclusion because the school will just point to the instances of "bad" behavior.

I think this looks like discrimination based on a disability.  Schools are supposed to support every student so they have assess to what the rest of their classmates have access to.  He shouldn't be denied a field trip because the school failed to child find and write up an IEP with the needed supports so the child's misbehavior is accommodated.