These meters are typically measuring AC emf, which in the FFE would be generated by the motor and AC inverter at the front end of the car. In any case, data concerning deleterious health effects of weak emf are equivocal at best. If it turns out that there are small increases in e.g. cancer risks, the main issue would be long-term exposure levels rather than very short term transient exposures to levels that are still fairly weak. IIRC, typical household background emf ranges from ~1-10 milligauss but certain high amperage appliances could temporarily spike that to much higher intensities (particularly those requiring close proximity while using - e.g. hair dryers and vacuum cleaners) - and exposures from these sources will likely be much longer than an occasional few seconds of braking.
I you're that worried about possible emf risks, focus on where the kids are spending the most time. E.g. throw out the electric blankets and move the child's bed away from the wall carrying a 50 amp heat-pump circuit. If the peak emf intensities in the car are really worrying you, then accelerate and brake as slowly as possible - but I don't think this would affect total AUC exposure.