We ended up using a crib tent when we discovered our son could get out - maybe 20?months. We let him watch us put it on and showed him how "cool" it was he was a big enough boy to have it...!
Introduced it on a Friday night, after keeping him up LATE so he was really tired and didn't have any 'fight' in him, and the darkness helped too. Then, at nap time in the afternoon, talked about it again with him,calling it a "tunnel" cause he liked tunnels at that age.
He really was fine with it and enjoyed giving one last kiss before the last 10inches :-) And giving smoootchies through the mesh in the morning.
It gave us such peace of mind, we'll definitely start to put ours on the baby's crib when he/she is 4mths old (before they're really aware of their surroundings). Our son was really 'athletic/coordinated', but our second baby may not be and could really get hurt.
VERY IMPORTANT: If you're child is able to take off their pants/PJs, and even MIGHT have an affinity to painting with their poo - no 2 piece PJs!! Our son quickly received the nickname: Poo-casso! If it happens, CLOSE the velcro tabs before washing it in the washer ;-)
Hope that helps!
In your case, if the goal is to keep the toddler out of the baby's crib, it might be easier to use the tent on the baby's crib instead. (Maybe that's what you meant in the first place and I misunderstood...sorry)