Rising 7 year old talking back/being rude

[SilentMammoth]

What the title says really. Nothing seems to have triggered it really, but I really want to nip this in the bud. I would love some advice, especially about consequences; time out tricky to enforce as she gets off naughty step when I put her there, I could put her in dining room and hold door closed but have 3 other dc under 4 so don't like being out of sight of them dealing with her iyswim? She doesn't have computer/ipod/TV time so can't take away that. I really want to bring her up to be polite and respectful but worry am failing.

ds (7) is talking back too, if its not too bad (its not fair type comment/bit stroppy) and depending on the mood he gets gently chastised/joked with to encourage him to stop the attidtude/do as told and this generally works.

if it continues or is bad (angry/really rude) he gets time out and knows when he's told to do time out its better to go otherwise there are other consequences until he does (I don't physically put him there as he's too big now!). he loses out on things he loves like bedtime story/chat, dessert, football, etc. he can sometimes lose up to three things if he doesnt do his time out when told, but has always gone in the end (still loses out on things if he didn't go when first told).

being consistent and following through on all consequences is only thing that works with ds. If your dd doesnt have computer/tv etc need to think what other things she will miss like activities, favourite toy, friends over, etc