"Those children are standing."
Translation:वे बच्चे खड़े हैं।
खड़ा (खड़े when plural) is an adjective and not a verb. You can't conjugate it like verbs. It just means that the state of the subject is that they are standing.
You can combine it with the verb होना('to be') to make 'खड़ा होना' but this verb refers to the action of standing up.
So, the present continuous form 'मैं खड़ा हो रहा हूँ।' means that I am in the process of standing up.
बैठना ('to sit') is a verb but it is used somewhat similarly. The present perfect form 'मैं बैठा हूँ' means that I am seated (which is what is usually meant by the sentence 'I am sitting'). The present continuous form 'मैं बैठ रहा हूँ' means that I am in the process of sitting down.