You can bet your bottom dollar that every single nut and bolt has been checked after the Smiler incident. Now is probably the safest time to go.
I went to Thorpe Park on fathers day (I don't habe a father or kids) and it was awesome. The longest queue was about 10 mins, we went on several rides multiole times too. Saw: The Ride was shut due to a safety concern raised after the inspections following the Smiler crash which was a bummer but still a great day nonetheless.
Also, tickets online are half the price of gate tickets, which is the price taken in to account on all those 2-4-1 vouchers so they're pretty pointless anyway. MSE has a whole section dedicated to Merlin Attraction tickets, simply search "mse merlin tickets" and you'll find it.
Have fun!