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:
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.