BJT vs Mosfet, switching variable current battery powered device
There are so many MOSFETs that will fit this bill. If you are sure that you want a low-side switch, then AO3414 is a reasonable choice, it will drop only a few mV at 50mA and is very inexpensive.

You might want to consider suing a p-channel MOSFET as a high-side switch, depending on the exact application.