ES is a professionals' market and therefore one of the most difficult for retail traders to prosper in.
CL, FGBL, TF, and all the currency futures are better suited for the retail trader despite the lack of deep bids and offers.
Best of luck.
If you have incurred additional overhead costs to trade for your clients than a fee might be appropriate, albeit a very small one.
The answer to your original question is yes, you can charge a fee and it is taxable income. Don't expect your clients to provide you with a W2, however.