Welcome to the hobby; playing with your kids is great. Mine started when they were 10 and 12; the eldest gave up years ago but the youngest still plays regularly when he is down from university.
Well done on buying a sensible first gun; play with it a few times to see what you think needs improving (range, accuracy, rate of fire, trigger response etc) and then start looking at upgrades. The Advice and Technical Help section of the forum is a great place to get advice and guidance on upgrades from some very knowledgeable folk.
One thing I would recommend if you do not already have them is a good pair of solid boots that support your ankles; surplus boots are excellent for this and quite cheap.
As for a pistol, I would highly recommend the CO2 powered ASG XP18 commander. My son and I have been using them for a while and have found them to be solid, reliable and consistent; CO2 is also more reliable than airsoft gas in cold weather, although they struggled at -6 degrees a couple of weeks ago.
Enjoy this silly hobby!