I've done it twice and I'm submitting my 3rd one next week. It's very straightforward if you have a genuine relationship, if you can show that you have enough money to cover the cost of the trip and, most importantly, there is a reason for the applicant to return to Thailand. The last one is the one that can cause a problem, if the applicant has no job or no property, and the sponsor lives in the UK.
Getting a UK tourist visa for your wife will only take around 2 weeks. She will have to submit the application and supporting evidence in person at VFS in Bangkok.
Children under 14 technically do need visas, however, they are not fined or banned if they overstay, although they do receive an overstay stamp in their passport.