Below is a picture of their headstone, which is quite difficult to make out.
I was able to find a mention of Robert's drowning in The National Republican, that was published on 24 Jul 1874.
Here is a transcription of the article from the newspaper:
Yesterday afternoon, about 2:30 o'clock, Robert Armistead, aged thirteen years, and two other boys, went in to bathe in the Eastern branch at the foot of Fourteenth street southwest, and off the old ferry wharf. While enjoying themselves in the water young Armistead got too far out in the current, which was running neath the surface and drowned before assistance could be rendered him. Efforts were immediately made to recover the body, which was accomplished in a short time afterwards by Mr. J. Moulton, and it was removed to the residence of the father, No. 1107 Ninth street southeast. Coroner D.C. Patterson was notified, but after viewing the body deemed an inquest unnecessary, as it was evident that death resulted from accidental drowning. The funeral will take place from his late residence this afternoon at 4 o'clock.