Fermenting lemonade can be a challenge due to pH being so low. There are two approaches that I know can work.
1. Ferment by using the yeast cake from a prior fermentation using a yeast like EC-1118 that is tolerant of low pH. These yeast will be more-accustomed to a harsh low pH environment. Then keep it warm, and aerated, and be very patient and it will usually get going.
2. Adjust the pH. It takes a lot of Potassium Bicarbonate or Calcium Carbonate to to bring the pH up from Lemonade levels (typically 2.8 or lower) up to the range where yeast can work, but if you get the pH up to around 3.2-3.4, you can typically get it done easily. Once the pH is adjusted, the yeast love citrus fruit. After fermentation is complete, you can taste it to see if it is tart enough to suit you; in most cases it probably will be as the acidity is still very high, and TA is more closely associated with acid taste than is pH. However, if you need to add some acid to suit you, using acid blend, citric acid, or more lemon juice will allow you to get the level you want. When I do this, I find the lemon aroma still present, but I do ferment them relatively cool. Adding some lemon zest will give you more.
Of the two approaches, I much prefer the second method. However, there are a lot of folks on www.winepress.us
that make a lemonade recipe called "Skeeter Pee" that uses the first method, and they mostly seem happy with the results. Check out skeeterpee.com
for the details.