Families, bullying and feeling very sad are some of the main things children are calling ChildLine about.
Newsround spoke to Angie Brown from ChildLine to find out more and to ask what kids can do if they're upset.
What is the main thing this report from ChildLine shows?
The main thing is that there's an awful lot of stress in children and young people.
The amount of children who are under 12 who have mental health issues is quite shocking really.
What are the biggest problems for eight to 12 year olds?
One of the main things is family problems. A lot of children feel caught in the middle when their parents are separating. They feel they might be asked to make a choice about which parent they want to live with even if that's not the reality.
Also other family problems, like if they don't get on with their brothers and sisters. Just because you are born into the same family does not mean you automatically get on!
Bullying is a big issue too. That can lead to children feeling really depressed.
Also eating problems are big too. We have spoken to some very young children about eating problems.
Or we speak to a lot of children who are just feeling left out generally and that they don't fit in.
Being clever can be negative too - clever kids can be seen as a bit geeky or a bit boffy.
What advice would you give to Newsround viewers who have problems?
The main thing we would tell young children to do is to talk about their feelings. Not to keep it all to themselves.
Children often feel that they can't talk to their parents in case they upset them. But parents and carers are adults, it's their job to listen to children.
If they feel they can't talk to their parents, they can perhaps talk to ChildLine or a teacher or the mother of their best friend, for example.
It's so important for them to talk. My main message would be not to bottle it up. Not to feel that people are going to judge you. If you keep things bottled up then they start seeming bigger in our minds and then they might get too big for us to deal with ourselves. It's so important to talk.
Have you noticed younger children calling in now than when ChildLine first started?
It depends what the problems are. With bullying, 10 years ago we did a special bullying line, I spoke regularly to children as young as five and six years old. We have this cosy idea of nursery school being so nice but it's not always true.
How do children know if they're depressed and not just feeling a bit sad?
Children don't use the word depressed. Children will call ChildLine and say things like 'I'm feeling really down' or 'I'm fed up all the time and I don't know why'. Or 'I can't stop crying' or ' I just don't feel right'.
Any child who thinks they have these problems should always talk about it to someone - ChildLine if they can't speak to anyone else.
What needs to be done to help all these children with problems?
Children tell us all the time that there's not enough counselling. And that is key in dealing with children's mental health problems. A counsellor can tease out from a child whether they might need specialist help or if they just need help to sort out one issue
So we think the government should give more money to provide more counselling for children.
Have you got any other advice for children with problems?
If you do feel desperate, one thing I like to say to children is that 'You never know what tomorrow is going to bring'. Yes it could be something worse but it could be something which is so much better.
My biggest message for children is to make sure they talk about stuff and not store it up. They should talk to someone they trust.
If you're worried about anything, you can phone ChildLine on 0800 1111.