Is Leaving Child in a Vehicle a lesser-included of Abandoning/Endangering a Child?

I found one case that says it is:

Millslagle v. State
81 S.W.3d 895

You need to look at that case in light of Hall v. State, 225 S.W.3d 524 (Tex. Crim. App. 2007). Also look at Article 37.09. Prior to Hall, all sorts of cases found offenses were lesser-included by looking at the trial evidence. The focus is now on the statutory elements of the requested offense as compared to the elements pleaded in your indictment, together with any facts pleaded in your indictment. I don't know the offense of leaving a child in a car.

I would say no. Leaving a child in a vehicle requires that the child (1) be left in the car (2) for more than 5 minutes (3) without anyone over the age of 14 in the car, and (4) the child was under 7. You don't have to prove pretty much any of that under abandoning/endangering -- you can have a child over 7, it can be less than 5 minutes as long as it was dangerous, and it doesn't matter if another child was with them. Different elements, not a lesser. Your case was pre-Hall, as John pointed out, so it doesn't control.

Now, if you were really specific in your manner and means in the indictment, then it's conceivable, but I can't think of anyone alleging abandoning/endangering in quite that way!