First of all, that's a wide age range. Psychological differences between kids of 5 and kids of 8 could mean thatthe 8 yr olds might find that activities thatthe 5 yr olds love are too babyish for them while the 5 yr olds may find that the activities for the 8 yr olds are beyond their capabilities. Just as an example : Are the 5 yr olds currently in school? have they started to learn to read and write in their own languages? For the 8 yr olds the answer will obviously be yes. But if the 5 yr olds can't yet read in their own language - can you introduce the written form of words which might completely contrast with all they're learning about reading in their L1.? But on the other hand, if you hold the written form back back, you're hindering the learning of the 8 yr olds who are ready for it.
I've written various books for primary level learners, but always year by year - taking into account their intellectual and emotional development and what they were learning at school.
Secondly, but just as importantly - do they learn English at school? If so there will be a huge difference in what the 5 yr olds know - probably nothing - and what the 8 yr olds know.
So this is a real problem - not just because you've never taught kids before but also because you've been asked to combine age groups with vastly differnt characteristics.
I can't help much with the second problem - I would try and convince the parents to separate them by age. and create at least two groups - 5/6 and 7/8 - but preferably 4. If it is a numbers problem I'd suggest that each child would learn more from fewer lesson aimed specifically at his/her level than in a longer course with kids who have incompatible needs.
For the first problem - you need a good coursebook. Preferably for the kids but at least for you. Never try and teach a course of a type you know nothing about without consulting books written by people who are experts in the field. In the coursebook you'll find games, songs, stories and all sorts of activities that are eight for kids of these age groups and levels.
I won't push mine
but any reputable publisher will have various series of materials for kids of this age group. See the young learners catalogues for eg
Pearson CUP,
OUP Longman or any other publisher you have faith in.
All these sites also have sections with advice for teaching young learners - lots of nice activities that you can use. But they do need to be inserted into a coherent course - so find out what the kids no already and choose a coursebook that's right for them.