The problem with this is when it is time to break things up, it may get violent, and a stampede could break out. Innocents are going to get hurt. Some NPOs will probably set up some shelters and tents to aid with the sick, with food distribution before the border. I wouldn't be surprised if a tent city pops up there, and Mexico has to deal with the mess.
And no matter which way this goes, Trump will receive criticism and condemnation. The only course of action for Trump is a firm one. The immigration debate is why the Dems are losing, and they will continue to lose until it is addressed properly.