Here are some options you could use, depending on the context:
* Informal:
* A bunch of toddlers
* A group of toddlers
* A gaggle of toddlers (this can be a bit humorous)
* A crew of toddlers
* More descriptive:
* A troop of toddling explorers
* A pack of playful toddlers
* A throng of energetic toddlers
* A swarm of tiny humans
Ultimately, the best collective noun will depend on the specific situation and the tone you want to convey.