i'd not bother with baths for the hatchling.
Bearded dragons are evolved for very dry country, and don't need access to standing water like a skink or a gecko would.
Plenty of fresh greens and chopped veg and live insects and he'll get all the
hydration he needs.
I rarely
bathe my dragons, then only if they've soiled themselves, are in shed and having a problem with some stubborn shed , or too hot (to cool down).
Are there trees in the garden ?
Take a skinny twig ( one that's maybe been shed by the tree and is now leaf free ), needs to be about 2 to 3 ft long.
Now when the hatchling retreats to the back of the tank , slowly reach in with the skinny end of the twig and try to gently get its end under the chest , arms of the hatchling , they tend to grasp twigs well if not scared by the twig.
Slow raise the twig with the hatchling holding onto the end and draw it closer to you , even to your other free hand or chest, and then place the hand over the hatchling to form a hand cup.
Harder if the tank requires you to reach in from above, all the dragon will see is looming very scary figure and 5 headed snakes ( arms and hands ) chasing it.
Try to have a disabled bug or other desireable food treat handy when you get the hatchling out , make a routine and ritual of this , and try to get it to associate you with a pleasureable experience ( food ).