That’s decay , they need to see a dentist.
first things first are you cleaning their teeth for them? Children can brush their teeth themselves but an adult needs to go over again, until they are at least 8 because they don’t have the dexterity to do a proper job.
You need to look at what they are eating and drinking inbetween meals and make sure that anything sugary/starchy is kept to meal times only. Every time they have even a sip or a bite or something sugary/starchy the teeth are under attack for an hour afterwards.
That’s not just sweets/biscuits/cakes but yoghurts,fruit juice, smoothies, raisins, flapjacks, cereals , etc. what they eat or drink at meal times doesn’t matter it’s the grazing inbetween that’s the problem.
Make sure, even if they brush their teeth afterwards , they drink only water , just before bed and at nighttime and they do not eat anything sugary/starchy just before bedtime .
Do not use the tartar remover , book a dentist. This tooth is supposed to last until they are 10 to 12 and it has only been in their mouth for a short time. They need to see a dentist but most particularly they need to change their diet.