Sandpiper is right....you're responsible to the organisers and you need to be guided by them. And I think you've actually raised two questions here - the right to photograph and the right to sell. You have the right to take photos as the official photog, and the organisers will probably have made consent to photograph a condition of entry for all children, but check that they have. When I've done similar events, if consent is not given, then the child does not enter. Making an announcement on the day will only complicate matters since it not only becomes unworkable but you will have difficulty verifying whether a person has the right to withdraw or give consent on the spot, and that will open a whole new can of worms. And what will you do if somebody claims that they didn't hear the announcement and so gave no permission for you to photograph their child? Or if Dad says yes and Mum says no?
Secondly, if you do sell photos later, then you won't be breaking any criminal laws (providing that you're not doing anything untoward or obviously illegal with them, either on your own or through a third person) because you were entitled to take them, having been appointed and so approved, and by having the required consent. That doesn't change. What might change though, is what you can do with them because they were taken for and on behalf of the organisers.....and so it goes back to you needing to establish what their take is on it.
What I would say to you though - and I speak as somebody who has been in exactly this position - is that having done all you can to check and double check, if you've still got any doubts at all, steer clear. Is it really worth enduring the sort of potential aggravation this could cause just to sell a few photos? Hope it works out for you.
Regards
Zin Zin