Are my trees stressed?

I have 2 baby tamarind trees for almost 3 months. I am watering them only if the soil seems dry none the less. I have noticed that on the leaves they have some stains which is worrying. I don’t know what to do.I normally keep them next to the window only if there are sunny days like today. Also i am thinking to feed them. I don’t know if I should do that now. Can you please give me some tips or any advice.

Looking forward to your reply.

Kind regards,