You can only pick sections of seats for away games rather than picking exact seats, ie for QPR the options to select will be UT Gold Normal/Silver Normal/Rail Seating.
Lower Tier seats are probably best for taking a young un. However, QPR is one where the majority of people will stand up for the entirety of the game, which is something to keep in mind. That said we’ve took the kids before and managed to squeeze them in at the bottom row.