Unless you are the parent, legal guardian, or someone with some other special legal duty to the child where this might be criminal neglect, yes, this is legal in, AFAIK, every US legal jurisdiction — there is no general legal duty to render aid.

In Germany it is different.

- failure to render assistance ("unterlassene Hilfeleistung") up to one year in prison or a fine

- Exposed to a life-threatening situation ("Aussetzung", § 221 StGB) – If a person leaves someone helpless in a life-threatening situation, they could be sentenced to up to 10 years in prison

Edit: Also note that murder would often give you 16 years in germany even though it is called live long.