The brown spots on the upper central incisors look a lot like enamel hypoplasia or enamel hypomaturation. Difficult to tell entirely from a photo.
Teeth that are affected like this (for whatever reason -there can be lots of reasons) are very, very susceptible to caries /decay and breaking down further.
It's vitally important to watch sugar intake. All sugar should be confined to one of the 3 mealtimes, as the pudding. Don't give your child juice /cordial or any other sweetened drink at all, but definitely never get them between meals. Regular teeth can 'remineralise' between mealtimes but if you're snacking on sugar between meals, teeth never get a chance to 'repair' IFYSWIM. Hypoplastic teeth, you have to be even more mindful.
Milk consumption-should be onto a lidded cup /beaker by now if FF. If using a bottle, stop that as soon as. Milk pooling in the mouth with a teated bottle will compromise those teeth further. Never let a baby or toddler with teeth fall asleep directly after the last night feed -be that FF or breast. You have to brush teeth with a toothpaste containing 1000 ppm Fluoride after the last milk at night for 1-3 year olds.
Raisins, most crisps, yoghurts, smoothies, other dried fruit, juice of any type (there is no such thing as sugar -free juice, no matter how it's described in the UK-it's all to mislead you about sugar content)-these all contain sugar. So remember to consume the above items WITH a meal not between meals.
Snacks between meals- bread + butter, veg crudities, fresh fruit (not in massive quantities as will cause erosion -another dental problem), cheese, sliced meats etc are best to eat between meals to reduce the chance of decay.
Good luck and make sure you brush your child's teeth 2 daily with age appropriate fluoride toothpaste. Remember, it's a non-negotiable activity of daily living when your child is uncooperative with toothbrushing. Make a game but if that doesn't do the trick, wrap in a towel and get on with it.