In my favorite of the Johnny Maxwell trilogy, Johnny crosses paths with Mrs. Tachyon, a bag lady who also is a time traveller. He and his friends travel back to WWII in their town with the predictable changing of time that must be fixed. However, instead of simply changing time back to its previous path, Johnny wants to prevent a bomb wiping out Paradise Street in the middle of the night. This necessitates a lot of maneuvering by the kids with the usual humorous Pratchett twists and turns along the way. I was surprised at what a page turner it became by the end as I stayed up way past bedtime to get Johnny and his pals home again. Here's a favorite bit of dialogue to tide you over until you can find the book which is out of print.
In the shopping mall, a joke was going wrong.
"Make me .. er," said Bigmac, "make me one with pickle and onion rings and fries."
"Make me one with extra salad and fries, please," said Yo-less.
Wobbler took a long look at the girl in the cardboard hat.
"Make me one with everything," he said. "Because ... I'm going to become a Muslim!"
Bigmac and Yo-less exchanged glances.
"Buddhist," said Yo-less, patiently. "It's Buddhist! Make me one with everything because I'm going to become a Buddhist! It's Buddhists that want to be one with everything. Singing 'om' and all that. You mucked it up! You were practising all the way down here and you still mucked it up!"
"Buddhists wouldn't have the burger," said the girl. "They'd have the Jumbo Beanburger. Or just fries and a salad."
They stared at her.
"Vegetarianism," said the girl. "I may have to wear a paper hat but I haven't got a cardboard brain, thank you." She glared at Wobbler. "You want a bun with everything. You want fries with that?"