It's normal for toddlers and even adults to get constipated from time to time and I wouldn't worry too much about it for at least a week.
If the toddler hasn't pooped within a week then you might start getting concerned especially if you're given the toddler some apple juice, prune juice etc and the toddler still hasn't pooped.
If the toddler hasn't pooped within a few weeks then you might want to speak with the toddlers doctor and let them know that you believe your toddler is constipated.
You can try increasing the toddlers fiber intake because sometimes that will help the toddler poop and eventually they should poop even if it takes a few weeks.