Map of London air quality http://www.londonair.org.uk/london/asp/annualmaps.asp
What you won't notice is any massive streaks running in and out of Heathrow (other than in the airport itself and the roads leading in). Aircraft pollution is dispersed in the air so exact flight paths make little difference.
Children in London (anyone for that matter) will be more seriously impacted by air pollution from road traffic.