We fly a lot between LHR and JFK and usually take the 9:30am from LHR to JFK and the 10:15pm back from JFK to LHR. We fly with Virgin Alantic if that matters. No problems with older daughter (2 years old) and little issues with our 5 month old.
Problem with the early morning flight back to LHR is that you have to arrive at JFK around 2.5 - 3 hours beforehand. This would mean a very early start for you all.
Would recommend a late morning/early afternoon departure from LHR (which will arrive in the US around mid to late afternoon) and an early evening flight departure say around 7.00/8.00pm back from either JFK or Newark.
Given the choice between the two I would choose Newark over JFK any day; JFK is a real dump of an airport.
You may feel tired going but travelling west is always easier than travelling eastwards as you "lose" time flying east from west. Children seem to manage with jetlag far better than adults. Also you would leave these shores in daytime to arrive in the US. Most flights back from the US to the UK are overnight ones.
Would take along enough of her favourite foodstuffs for the flight as childrens meals can be poor in quality as well as being served not long before the adult meals. Also her favourite soft toy if she has one along with a change of clothes in case there are any accidental spillages.
There's a morning flight from JFk to Heathrow with British Airways which we did. It leaves around 9 30 am I think and you arrive back at Heathrow in the evening UK time. Travelling all through the day definitely minimised jet lag.