Back when I was teaching Sunday School we did a project  that has stayed with ME.
And since so many of you have Bible studies in your daily schooling, I thought I'd share it.
I'm planning for us to do it again this week.

It's messy.
It's grace-filled.

You can use more of the following Scripture for your older kids. And just the specific verse for the younger. And you can use any or all. They all tie-in together. And that fact alone makes it seem extremely important in my mind.
Matthew 23 (specifically verse 25)
Mark 7 (specifically verse 14)
Luke 11 (specifically verse 39)

You'll need:
Styrofoam cups, 2 per child (but any container will do).
Dish detergent.
Water.
Dirt.
I do this outside...

Get both cups wet. Completely wet.

Add mud to the inside and outside of both cups.

Wash the outside of one cup. Just the outside. Note the mud inside.

Then wash the inside of the other cup. Careful to try and keep that mud on the outside.

Ask which cup they'd rather have a drink from (if they could only have a drink from one of the two).

We incorporated heart lessons, appearances (concentrating on), judging ourselves and others by what we see, "outwardly" and "inwardly", whitewashed tombs, and most importantly we noted how cleaning the inside first made the outside cleaner, too, as it overflowed while pouring. But it was really easy to clean up the mud from the outside and still keep the mud/filth inside.