You mention its your 3 1/2 year old you worry about.
Will they be starting school in September, or the following year? Don't have any kids myself so not sure if its still the way to be in school aged 4, or if most now wait for 5.
Is it possible to slide your shift? for example, start work at 7am, finish at 3.30 or 4? This gives you a little more time in the afternoon. It might mean collecting them from the creche earlier and having more time with them during the day.