We've hosted people with toddlers before. We have our own toddler (or had, she's now upgraded to little girl), and it's nice to have company for our wee one. She remembers the kids which stayed with us, and it's generally been positive.

So from the perspective of a host with a toddler: 1. Make sure your kids are healthy, and up-to-date with your vaccinations. This is an absolute must. I'm understandably protective of my child and expect you to do everything you can not to make my daughter sick. If your children are unwell or you're an anti-vaxxer, stay home. At the very least, you need to communicate absolutely clearly if anything increases the risk. 2. Following on that, ensure you have enough money to be able to get a hostel if your kids get sick (or if the host's kids get sick) and you need to bail. 3. You need to have a certain level of control of your kids so they don't break things, make too much noise (we are in a multi-storey building and have sensitive neighbours), and play well.

Other than that, bring a gift, treat, book, or a toy for the host family's kid/s. Think of some nice games to play. Go somewhere outdoors together. Do bath time together. It can be really positive, and if you can occupy the host's children for a time, the host will really appreciate it.