子どもに、「ズボンあげなさい！」って英語でなんて言うの？

You can say either of these examples and you can demonstrate how to do it. I look after a small child and I will say, "Pull up your pants!" nicely. It is a simple sentence to say to a child and after demonstration they will understand. Repeating is also key, repeat "Pull up your pants." a few times so it will sink in.

Well, there are two points to make. 1. In the UK, 'pants' are under garments. We say 'trousers' on this side of the water. 2. It depends on the age and intelligence of the child you are addressing, how subtly or otherwise you inform them about this issue. You could just give an order using the imperative form: "Pull up your trousers!" Probably this would be appropriate for a persistent offender. For a more mature child, it may be enough to ask if they are wearing a belt. They will deduce that you have noticed their trousers are falling down - and hopefully address the problem

The UK English word for pants is "trousers". When they are hanging down and not on properly we tell people to "pull your trousers up!" When something is not correct and in the case of clothing not put on properly we tell people to "sort it out". This means to fix it and make it proper. In this case you could say "Sort your trousers out!"

When you're trying to tell a child to pull up their pants, then you can say: -Please pull up your pants. -You need to pull up your pants. -Fix your pants. They need to go up one.

To tell a child to pull their pants up is to tell them to straighten their clothes out. It is kind to say my love, or my dear, or my sweet, at the end of the sentence to show love and kindness.

Properly means into it proper, or normal state, orderly, etc... For example: She stacked the papers properly on the table. "Pull your pants up properly!" and "Please pull up your pants." are both ways to ask someone to pull their pants up.