Separation anxiety is completely normal in younger children and shows they have a secure relationship with their parent but some children will find it harder than others to learn to separate when starting school or nursery. A useful explanation, some top tips and a video can be found here and in this great video on ParentChannel with some interviews with parents and children:
The Invisible String by Patrice Karst. A picture book on separation anxiety (or loss) that is excellent for ages 3-7 yrs. Find it here
A Boy and a Bear by Lori Lite. Children's relaxation book ideal for ages 3-4 yrs , can be used with a small group of children e.g in pre-school or nursery too to try and teach some simple breathing techniques. Find it here
Relaxation for young children: 'Relax like a cat' .Follow this moodcafe link for an illustrated step-by-step relaxation leaflet which also can be downloaded as an audio-file. Takes young children through sequences of breathing, tensing and relaxing muscles by imagining themselves to be a cat!
Sesame street “Breathe” App (US): This is a lovely little interactive App for dealing with common toddler/younger child scenarios which can lead to worry or frustration e.g. feeling anxious about going to nursery/school or getting frustrated putting on shoes, by teaching kids to stop, take a breath and think of solutions. Choice of Spanish or English. You can read a review of the App on 'Common Sense Media' here. It's free and available for Apple or android.