おやつの前に片付けようねって英語でなんて言うの？

Both of these phrases can be used to express the idea to cleaning up before eating snacks. They have the same meanings and also express this idea in a light and fun way by using words like, "snack time," in the form of a special event and the verb, "to enjoy," to express the enjoyment we can have after cleaning up.

You want to put across the point that it would be better for the child to clean first.
Use the snack as an incentive to get them to do it.

1. To do things together as a suggestion is very soft and kind.
2. Using the 2nd person plural "We" is very inclusive and softens the instruction. The imagery of a 'nasty germ' may also appeal to a youngster.
3. To elicit the response from the child will give you an opportunity to give praise and confirm the child's good learning.

Show me how good you are at cleaning up then we'll have a snack.
Meaning= this is getting the child to show off , which actually gets the tidying done.
The snack is used as an incentive after showing off their cleaning skills.

Make sure (your room) is looking beautifully tidy before we have a snack!
This is a bit more gentle than no 1 and is a bit more vague.
If it isn’t their bedroom they’re tidying you can change the words with brackets around it.

How fast can you tidy up? Winner gets a snack !
Meaning = getting them to tidying up as fast as possible so it turns into a game.
The snack is the prize. Inserting a bit of competition always gets children going!

These are really gentle ways of telling a child to tidy up.

Lets put everything away then we can have a sandwich/etc. - This is like an agreement with the child to clean & tidy up and then they will have their snack. But it comes across gently.
We have to tidy up before we have a snack. - I would use this often, as it will become like a routine. The child knows that after they have tidied up they will have their snack and then once it has become routine it will change to "Time to tidy up!" which you can shout from another room if so and still comes across gentle. However once they know the words "Time to tidy up!" they will know a snack or a treat is coming.

tidy - arranged neatly and in order
snack - a small amount of food eaten between meals
You first need to tidy the room/area and once you have finished that you will get a snack.

"You need to clean the before you get a snack". The room needs to be clean and then a snack will be given.

1. Please clean up your room before eating snacks.
   スナックを食べる前にお部屋をきれいにしてください

   2. Please put away your toys before eating snacks.
      お菓子を食べる前にお子さんを離してください。

   You can use both of these statements to politely ask your child to clean up before eating snacks.

Snack - a small amount of food that is eaten between meals, or a very small meal. The first sentence in this case is a more directive suggestion to do smth first and to get a reward afterwards. The second sentence is a question which they could choose not to follow.

First - before anything else.
Often when asking a child to do something not fun (like cleaning) they don't mind doing it if you are helping them. So saying 'Let's clean up' means that you are going to help them. (Let us clean up) It also sounds more cheerful and gentle than saying 'Go clean up!'.