Marcus , you are probably referring to the Bargehouse Causeway slip ( which appears on the Boatlaunch Site already referred to by Geoff ) .
This is probably not a slip for beginners ( with young children especially ) . I would suggest that this slip is only of use at slack high tide , using a 4X4 .
Gallions point Marina , a few hundred yards away has it's own slip , from which you can lock in and out of the Thames except low water +/- 1 hour . ( Assuming of course Marina is open ! It is not 24/7) .
The Tidal Thames may not be the best place to practice your new interest with a young family . With tides of over 7 metres , fast flowing at times and various other fast craft about , I would give it a miss unless I was in the company of other ribs/sibs , and knew exactly what I was doing .
Regards & Safe Sibbing........ Bern