In general in the UK the breakfasts in the restaurant are much more complete than in the lounge. It's an interesting point about the extra charge for a child in the restaurant - which I hadn't really considered before - but as a rule I always eat in the restaurant. The other big advantage of the lounge (other than for your child) is if you want to eat breakfast at peak time (say later during service at the weekends) where the restaurant will be crowded - usually the lounge is very quiet.