I went to The Gambia on my own, but didn't enjoy it. The sexual harassment was horrendous, much worse then I had anticipated, and I found it very difficult to deal with. Going as a family may be easier!
You'll need your yellow fever jab, and have cash ready for the airport visa. Definitely use lots of mozzie spray, and be aware of sandfly on the beach.
Wash hands before food, especially if you've handled money as it was particularly grubby. Avoid salad etc - the usual issues with water apply. I did get a dodgy stomach which lasted a fortnight after my return, I think from forgetting to wash my hands and eating BBQ meats with bare hands. Silly mistake.
I had my details stolen from my checked in luggage on the return flight, and was deluged with handwritten begging letters for months!
Beautiful country, amazing weather, wonderful wildlife, most people are lovely but the bumsters are an absolute nightmare.