What is the verb for making objects on a plate jump up and down?

It depends on what the objects are, the thing they're located in, and how many times it is done.

If it happens multiple times, you might be bouncing the objects:

[Merriam-Webster, from bounce]
transitive
2 : to cause to rebound or be reflected
    // bounce a ball
    // bounce a light ray off a reflector

intransitive
1 : to rebound or reflect after striking a surface (such as the ground)

// He was bouncing a tennis ball against the garage door.
// bouncing the ball back and forth

In this example in the sentence, in is not the correct preposition. There are two others you can use:

• I am bouncing the objects on the plate.
• I am bouncing the objects against the plate.

It depends if you want to emphasize where the objects are (on) or what they are rebounding from (against).

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But if it's dice on a plate, and you do it only once, then you would say the following:

I am rolling the dice on the plate.

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If what you're describing is cooking, then you would use a different verb and prepositions, and it would be somewhat contextual.

• I am flipping eggs [with a frying pan].
• I am flipping pancakes [with a frying pan].

      but

• I am tossing salad [in a bowl].

Generally, you flip things with a frying pan, but you toss things in a bowl.