It sounds like you won't be working with any this young, but when working with very young children (5 years and younger), here are a few things to add to your kit:
- hand sanitizer, and lots of it
- Airborne, Emergen-C or whatever you take to boost your immunity... and lots of it
- baby wipes - for messy mouths, snotty noses, etc
- a couple of pull-ups diapers (those transitional diapers toddlers wear)
- a couple of extra pants and extra shirts that have flexible fit. If the kid has an accident, these will save everyone's day
- a few gallon-size freezer bags for storing the clothes the kid was wearing when s/he had an accident
- plenty of fun band-aids (Spiderman might be nice right now)
Also with very young kids, you need to prepare them for aspects of the theatrical environment that they might find frightening. They might find the darkness backstage scary, so take a minute early in rehearsals to sit with them, show them around, introduce the scary biker stagehands (who are always nice as can be), and reassure the kids that they are safe. They might be scared by the bright lights, so practice bringing up the light levels while they're onstage (if you can). They might be terrified of the soprano screeching her aria in their ear while she clutches her child actor - so make sure they spend time with the performer beforehand and that she explains how she acts and sings. Again, reassure the child that they are safe and be sure that they feel comfortable. Make sure the child knows what to expect. Let them ask questions and give them honest and informative answers. Also take time to talk with the child about reacting (specifically NOT reacting) when the audience laughs. Be sure the child knows where to look or whom to watch so that the kid isn't gawking distractedly at anything on- or offstage. Give them focus and they'll do well in performance. If there is violence, cursing, blood, or death onstage, talk with the child about it - ideally with the parents present - and reassure the child that no one really dies or is hurt in this theatre. We are pretending. Kids do understand this, but they get scared.... well, when the acting is convincing.
But in general, keep your patience and watch your language. Kids are observant and inquisitive, and might like to ask questions of you constantly. If you give them respectful, simple answers, they will appreciate it and generally cooperate with you. Take the time to explain why they have to stand in a line when preparing for their entrance. Be ready to check shoelaces, glasses, bubblegum when preparing them. Remind them that safety is important backstage, and assure them that they are in a safe place. Keep your sense of fun and your calm demeanor.