Yoghurt or yogurt, is a dairy product produced by bacterial fermentation of milk. Any sort of milk may be used to make yoghurt, but modern production is dominated by cow's milk. It is the fermentation of milk sugar (lactose) into lactic acid that gives yoghurt its gel-like texture and characteristic tang. Unsweetened yoghurt is healthy provided low fat milk is used. Growing children and teens may or may not need the vitamins and other food value in whole milk, research is currently unclear. 
Yoghurt with high Sugar content isn't healthy.
Fat free doesn't mean sugar free, especially when it comes to yoghurt. It's often laden with sugar to keep flavour and texture when fat is removed. A 150g (5oz) serving of some 0% fat yogurts can contain as much as 20g (0.7oz) of sugar - the equivalent of five teaspoons, says Action on Sugar. That's getting on for half of a woman's daily recommended intake of added sugar, which is 50g (1.7oz). It's 70g (2.5oz) for men.The fruit in yoghurt you buy is as best as healthy as dried or tinned fruit, the food value deteriorates after the fruit is cut up and put into yoghurt, and there is almost certainly added sugar. If you feel like yoghurt with fruit when you're at home why not try cutting up some fruit, mixing it with yoghurt and eating it at once while the fruit is still fresh. It tastes good unsweetened, it also tastes good unsweetened with sweet apples that I bought for my favourite, chocolate pudding. (You can of course sin just a little and add a small amount of sugar, honey, syrup to the yoghurt. Hardly any of us have the will power to resist all those tasty, unhealthy foods. The home made yoghurt with fresh fruit will still be healthier than bought fruity yoghurt and you know how much sugar you added.) If you're preparing packed school lunch for a child some fresh fruit that hasn't been cut up and some plain unsweetened yoghurt to eat with it is healthier than fruity yoghurt. 
Soy yoghurt is a fermented product made from Soya beans and water that's similar to yoghurt. Unless you read the label you probably won't tell from the taste which is which. Soy yoghurt is also healthy provided it's unsweetened.
You need to take care over hygiene. Here is what a website that understands food hygiene recommends. "Store leftovers in the fridge within two hours of cooking and eat them within three days." Also foods like milk, yoghurt and tinned food should be eaten within three days of opening the container. Plan how to use a large carton of yoghurt within three days before you open it. If you end up with yoghurt that's been open too long put it into a dish early during cooking and boil it thoroughly.
For many/most people cheese is likely a more convenient staple than yoghurt as it can be stored safely in the fridge for longer.
- ↑ Whole Milk Or Skim? Study Links Fattier Milk To Slimmer Kids
- ↑ Sugar: Five foods surprisingly high in sugar
- ↑ This can be troublesome, fruity yoghurt is advertised on some websites where kids go to play video games. After they've seen the adverts the kids likely pester their parents for that unhealthy yoghurt. Young children often do not know how deceitful advertising can be. Parents may need to explain that they shouldn't trust advertisers who pretend a product is healthy. Parents need to be firm, give in to pestering once and there'll be more pestering later. You can be firm, tell the child, 'No you can't have that sweetened youghurt too many sweet things are bad for you.' and stick to it but that's a bit traditional. (Too many restrictions cause resentment so later when the children can choose they could turn against healthy food that's been forced onto them and eat all the wrong food.) For older children who can choose a more liberal approach could be to allow the child just one sweet item in each packed lunch, say a small bar of chocolate, a scone, a donut, a few sweet biscuits. Then you can let the child choose, tell the child the yoghurt will be instead of some other sweet item. Again you must stick to this, don't let the child pester you for more than one sweet item.
- ↑ SafeFood