You saw how poor russias army turned out? Well the Belarusian one is worse. It's got a tiny budget, not many troops, it's equipment is even older than russias and it is conscripts based. If ordered its unlikely they fight.
Plus Lukashenko only retains power by having the army around. If that goes, he is in trouble.
It'd be a slaughter. The Ukrainians have been preparing extensive defences along the border, are much better equipped and battle hardened.