That's 4 travel days.
Do check this against booking normal advance tickets, taking into account the pass reservations you will need (except for Amsterdam - Antwerp, which has an hourly reservation-free intercity service).
Forget pass-just book advance direct, I assume you also book HTLs etc fixed-so you can do trains same. Saves the hassle+nerves for all these seat-only bookings too. If you wont believe-just compare prices.
IF you book at least 7 days advance the normal hourly trains Ams-Antw are cheaper and even stil without fixed timings-can use any of these trains on the day.
Hint: €* says it offers many seats but for RETUrn only from LON to Belgium (from 78€ for the 2, just a little more as the supplmt you have to pay with pass-get off at LIlle onward and use TGV to Paris from there (and maybe even see this town for a few hrs).
P→ Ams is thalys.com A,ms-Antw is nsinternational.com or also b-europe.com
For child it all depends on age.