Should i be worried about my dog doing this?

Yesterday, in the park a little 2 year old came running towards my dog, Tia (the dog) ran up to him and stopped with the hair raised along her back, i was so worried. Tia's never showed any sign of aggression, thank god as i have a 4 month old daughter, but she can be wary of kiddies running around her. What should i make of this?