If it's a nice day and they are landing on 27L, you can always go to The Green Man pub. It has a nice beer garden for the kids to play in, and you yourself can have a pint as the planes fly overhead.
If you go down the A30 to Hatton Cross, turn right and it's just around the corner on the left.
Alternatively, there is a street called Myrtle Avenue that is popular with the spotting folk, which is about 5 mins walk from The Green Man.
Google Maps
(The Green Man is at the junction of Faggs Road and Green Man Lane on the map above, and you can also see Myrtle Avenue).
If they are landing on 09L, drive along the A3044 and you can park on the pavement at the side of the road.
Google Maps
(On the map above, where it says Stanwell Moor Road is where you can park - if you go to the roundabout and turn right through Longford village on the Bath Road, there is also a Mcdonald's where it joins the A4)
For 27R landings, you are best going to the old Visitors Centre, which is now called Heathrow Academy or something like that.
Google Maps
(The car park is at the end of Newall Road in the link above - Mcdonald's is the small white building just across the A4).
