You already tried to kill them once. Just finish the job instead of trying to revive the maimed victim.

You will be better off planting new 5-gallon crepe myrtle trees where they need to be and digging the old ones out - dig down and chop through the roots - and discarding them.

1 - They will take YEARS trying to recover from the damage you did when you chopped them to the ground. Your chances of getting a trunk are very slim.

2 - You will have a very hard time digging a big enough root ball for a 10-year old tree. If they were priceless specimens, yes, but obviously you didn't think so.