Johnson & Johnson makes a mosquito repellent for babies - you will find it in the baby section of most drugstores - rather than the repellent area. It's in a green bottle. I used that on both of my children from about 6 months. You can also invest in a mosquito net for your buggy if you have an infant.
I've since moved on to Mozz-Away for my 4 year old. It's a citronella based repellent and seems to do the trick for him - though it must be reapplied every couple of hours. I've also used the patches for both of them - but they tend to be pulled off either by each other or friends - so best to double up