Stupid questions about transit. Especially the Red line.

I am planning to come to Houston with my kid (a 4-year-old) and planning to ride the metro red line to get to places because he is absolutely nuts about trains and buses. The thing is I have not used a rail or bus in decades.

I am planning to use the mobile app for paying for fairs etc. Do you guys think that will work well?

If it does not work are there ticket machines that are easily accessible? do those work with a credit card?

Is it safe to ride with a kid?