Digging near hedge

Hello, we have an emerald cedar hedge that's about 15-20 years old (our guess). The trees are not the healthiest/fullest as we think they were affected by insufficient watering in dry summers by the previous owners. We would like to plant a few shrubs along the hedge line to add some interest/depth and hopefully help fill in the bottom for more privacy. Does anyone know whether we risk harming the trees if we dig holes (for planting) about 3-4 feet away from their trunks? I think these trees have shallow roots and I don't want to hurt them! Thanks in advance for your insights!