IR will do fine, no problems there.
To give peace of mind you can do a test shoot before the actual shoot takes place.


The group will probably be about 6-9 m wide and 3-5m deep
2x 500 Ws with umbrellas is not overdoing it.
I would also take a pair of wide direct reflectors like the BW1885 to
lighten the scene with direct light if the output from umbrella's is too small.

A higher point to shoot can be very useful.
Bring a small ladder for this purpose.