Look at this picture
When I tell my toddler put the straw into the milk box, she often push it completely into the box. And as a result, the straw got lost inside the box.
What should we express in this situation?
Please push the straw halfway into the box! (but sometimes we need to go two-third of the straw not just halfway of the straw in order to reach the milk).
Please do not push the straw into the box completely!
It is idomatic to talk about pushing something partly or completely into something else.
Insert a garlic clove into each hole and push completely into the meat.
However, I would note that "do not push the straw completely into the box" is rather adult language to use to a toddler; I would rather say 'don't push it all the way in!". Also those cardboard containers holding milk, juice, etc, are often called cartons rather than boxes.
If you want to express such an idea, I'd recommend: "Keep (hold of) this (drinking) straw that it won't sink in the box."
The reason is that the children of the age from 1 year old to 3 years old are more inclined to percept some verbs in comparison to others. These children have not any knowledge about the majority of idioms, proverbs and sayings which they usually take into their passive and active vocabulary after they reach the age of 5-6 years old.
The phrase 'push something halfway' is not for the toddlers' usage completely who learn to drink with a drinking straw.
As far as I know there is no such idiomatic expression, because of the special straws for the sachets (packets) with child food. It seems to be useful: https://www.arktherapeutic.com/blog/how-to-teach-straw-drinking/
