Eureka is fabulous - but tends to be very busy and very noisy! But it is easy to get to by public transport, being right next to the railway station.
I'm not sure it would be ideally suited for a meet-up, as it tends to be so busy and so noisy, and the kids do need a fair degree of supervision. But if it were a nice day there would be scope for a massive picnic together on the grass outside next to the station.
Costs: U3s free, adults & children £6.50 each.
Saver ticket - 5 people for £27.50.
But if you send off for Tescos museum vouchers you can get £10 worth of entry vouchers for £2.50 clubcard vouchers.
You can also use clubcard vouchers in a similar way to get into MAGNA at Sheffield, which is great fun too.