Hi! I think they are so young to mix tenses if they are learning a 2nd language. Anyway, they need more practise in order to get used to them. One of my favourite exercises to do is, at first, separate the different forms and do the next activity:
Affirmative, Present simple: Sarah plays football.
Affirmative. Past simple: Sarah played football.
Affirmative. Future simple: Sarah will play football.
... and so on.
Next, they should try to do the same in negative and interrogative forms.
On the other hand, a more motivating way than this is to use wordcards with each part of a sentence (the subject, the verb, etc). So, students will order them to create correct sentences.
One card: She. Another card: plays. Another card: football. (the same with more tenses). We mix them and they form sentences in the correct order.
Hope this helped!
Ana.
