Baby fire-types are always a bit of a challenge, anonymous, but in the case of chimchar, it may be manageable. You see, chimchar, as a primate pokémon, possesses a decent level of intelligence, so if you try to teach Hestia something simple, she may have an easier time grasping it than, say, a cyndaquil.
Or in other words, when she lights something on fire, turn her around and show her. Have her help you put out the fire and explain to her that the fire came from her. Train her a bit as well so she understands how to consciously use her flames, and teach her why she can’t simply set everything on fire in the same way you might teach a small child why they can’t do something dangerous.
Sometimes, trainers also find it helpful to give their chimchar a pokémon doll to carry around when they’re young. If their chimchar succeed in carrying it around without setting it on fire, they’re given a small treat during meals. If not, they have to put it out (and if they do this on their own, they might get a treat, although this might also encourage some chimchar to set it on fire in the first place). But by giving a chimchar something tangible and theirs to care for throughout the day, they gain a better grasp on their abilities and learn a bit quicker to control their back fires.
Most of all, though, be patient. As a baby, she’s prone to get into mischief, but by always guiding her with a gentle but firm tone, she’ll learn, little by little, as she grows.
Best of luck, anonymous!