Used pianos aren't that much. 6 and 8 year olds aren't going to care. Just get them playing.

You can get a used piano for free and put 5 or 6 hundred into it and have something decent for them. The small spinets are fine for that and aren't that heavy.

A 400 digital piano is going to be something you need to go try playing, have your kid play it and see if they are excited.

I would prefer a used spinet to a 400 dollar digital but its more about your needs. I wouldn't stress about this.

You can also rent pianos, they'll bring to your house and then take it away.

I second the others that you need the weighted keys and some pedal action. So stay away from a "keyboard."