The red brick does not have a high "clutch power".
We often say "I can't fit in the children's playhouse."
When we say this we mean that we can't fit inside the playhouse. For LEGO pieces, you would say
This red brick does not fit tightly with the yellow one. (Only parts of a piece go inside another one, not the whole piece)
About "flush fitting" and "snaps", I agree with what Jack said. It could mean that the "studs" on the red brick does not go into the yellow brick (because the "pipes" on the bottom of the yellow one are too big), or that they are too loose to "snap" together. So, "snap" could mean either too tight to fit or too loose to fit, and so it does not work for your case.
In your case, the LEGO bricks are coming off, or they are falling apart because they are a bit loose. In this case, the technically correct terminology is "clutch power".
... 'clutch power' which is "the ability of its bricks to snap together tightly while also being easy to separate, thereby readily allowing for de- and reconstruction" — How much usage can a LEGO piece take before it loses its 'clutch power'? in Bricks SE.
The key is "clutch power," which is the grip that holds one piece to another. — Lego's Market 'Clutch Power' in The Washington Post.
You could say
This red brick has lost its clutch power. Or This red brick does not have a high clutch power.
If you think your child might not understand that, or if you want to use words that are not technical, you could simply say
These bricks don't stick together firmly, they keep falling apart all the time.
Here is something from the LEGO website:
Have you ever wondered how LEGO® bricks stick together and not fall apart? ...
... bricks have a much higher “clutch power” when there are evenly spaced tubes on the inside of each brick. The tubes on the bottom interlock with the studs on top of other bricks. The studs get neatly wedged in between the tubes and the sides of every brick making them stick together firmly.