Under 5s aren't yet being vaccinated. It's due soon, however.
I tried to look into the gap between infection and vaccine including on the JCVI website. Nowhere would give a clear answer as to why except for in 12-18yos they felt that the 12 week wait would lessen the chance of the already rare myocarditis in that age group. The wait for adults between infection and vaccination was 28 days but there was no mention of this being because the vax was pointless to have in this time period as indicated above. I don't think it's a reach that Grace would have needed approval from her team to have it, tbh.
But... She's still not had it regardless of all of that which is really bloody infuriating.