How can I keep strangers out of my preschool to protect the children?

Keep doors that enter the playground and the front entrance locked. Use a buzz-in security system and only allow entrance to authorized people. Make tradespeople and delivery people wait until you have checked who they are. Always ask parents/guardians for proof that they are allowed to take the child home with them when it is time to leave; this can be done by having them sign forms saying who can collect their child (e.g. mom, dad, grandparent, aunt, etc. but nobody else). Get to know the parents really well so you know them by sight and name.