[AussieBird]

One idea: hand out a set of number strips where the HTO pieces overlap and ask children to quickly cut them out (or do this yourself as time is so short).

Then you first of all, and chosen children after you, read a card ( attached) eg what number has 4 in the hundreds, 3 in the tens and 2 in the ones . The class finds these from the strips they've cut out( 400 30 2). Ask them next to overlap the pieces so they see just one number (432). Reinforce that 432 is 400+30+2 and vice versa.
Do more hands-on examples like this.

Finally ,children could show their understanding of the concept by doing a worksheet, using the same overlapping number strips ( attached) or play a game ( attached)- you'd be best to laminate a copy of the gameboards for pairs ( I've attached 2 smaller versions per sheet).

20 minutes is not long enough! Good luck!