Flying a kite can help your child understand the Holy Spirit.
The best object lessons are often normal things. Jesus used wind to help Nicodemus understand how the Holy Spirit works to change people's lives (John 3:5-8).
There are many times when you can use the wind to open a discussion about the Holy Spirit. Here are just a few:
1. When flying a kite together
2. When untangling your daughter's hair after it's windblown
3. In a windstorm (added benefit of distracting the your kid from the storm if he/she's frightened by it)
4. When a tree or leaf blows
Once you've drawn your child's attention to the wind, you can ask if he/she can (or could) see the wind (help them understand that, no, they cannot see it; only it's results).
Then ask them if the wind is strong (yes, very strong. Point out the kite pulling on the string, a broken branch or their tangled hair etc).
You can then talk to them about John 3:5-8 and how the Holy Spirit wants to come into his/her life and make him/her a new, saved, person.
You may also want to share Galatians 5:22-24 explaining that when the Holy Spirit is invited into his/her life He will grow those good character traits into their lives.