Sound advice from the posters above.
One can only echo the sentiments regarding sending someone on their 2nd, 3rd, or even 8th solo flight - unless you are happy then do not get out. I personally have flown with one particular student three more times following their first solo before I was happy to let them go again. Fortunately, most students know the score and have a decent sense of their own vulnerability therefore it's rare you will face confrontation with such action.
Consistancy is what yer looking for and as Keygrip wisely reminds us - 3 consecutive good circuits and all things remaining the same, ya gotta let them go.
Enjoy instructing and don't get suckered into flying when the weather is closing in or the aircraft, it's equipment or the instrumentation is on the fritz!
VFE.