Economic evaluation of early childhood interventions is challenging because it is hard to extrapolate the full range of long-term benefits and public cost savings from short-term effectiveness evidence. It is also cumbersome and expensive to gather long-term evidence. One way to address this issue is through the use of microsimulation. This paper introduces a childhood microsimulation model, LifeSim Childhood, that is capable of extrapolating the long-term effects of many childhood risk factors on a broad range of health, educational, and social outcomes and public cost savings up to age 17 in the UK. It is based on bespoke modelling of longitudinal birth cohort data. The aims of this paper are to describe the general modelling approach and methods underpinning LifeSim Childhood, to present a simple illustration of how the model can be used to simulate the effects of early childhood poverty, and to compare our illustrative results with estimates from quasi-experimental studies. Our model is based on analysis of data from the Millennium Cohort Study which follows children born in the UK around the year 2000. Our causal inference strategy is to focus on causal pathways that can be explicitly justified by existing inter-disciplinary scientific knowledge and to estimate the total magnitude of long-term causal effects by controlling for an explicitly justified set of confounding variables. As well as describing our data inputs, regression analysis methods and simulation methods, we illustrate how the model can be used to evaluate four hypothetical income-shifting scenarios in early childhood and estimate the general magnitude of long-term public cost savings and wellbeing benefits, alongside a battery of more specific outcomes.