Hill Child was commissioned as a second lieutenant in the 3rd Battalion Royal Scots, a part-time Militia battalion, on 25 October 1899. The battalion was embodied for full-time service in the Second Boer War on 5 December that year, and in early March 1900 left Queenstown on the SS Oriental for South Africa. They landed at East London on 21 March 1900 and by July was engaged in operations against Boer Commandos in the Transvaal. Hill Child was wounded, and returned to the United Kingdom during Christmas 1900. He was promoted to lieutenant in the militia battalion on 6 March 1901, but in July was commissioned into the Regular Army as a second lieutenant in the newly raised Irish Guards. Promotion to lieutenant in the regiment came on 1 March 1902, and he was chosen to carry the colours at the first presentation of Colours to the Regiment on 30 May 1902, following which he was appointed a Member of the Royal Victorian Order. He retired from Regular service in 1909 and was placed on the Reserve of Officers in 1910. On 8 February 1910, Hill Child was promoted to Lieutenant-Colonel in the part-time Territorial Force and was appointed commanding officer of the II North Midland Brigade in the Royal Field Artillery. He was in command when the brigade was mobilised as part of the 46th Division in the First World War and served with it on the Western Front. The brigade was later numbered CCXXXI. 46th Division saw its first major action at the Battle of the Hohenzollern Redoubt in October 1915. Hill Child was awarded the DSO in 1916. At the Battle of Gommecourt on 1 July 1916, organised as a diversion from the main Battle of the Somme, the divisional artillery was allocated the task of wire-cutting: CCXXXI and another brigade formed the Left group, under the command of Hill Child. This group supported two battalions of the Sherwood Foresters, but the German wire entanglements were in dead ground and could not been seen by artillery observers. The attack was a costly failure, and Hill Child was a member of the court of inquiry into the circumstances. On 13 March 1918 the Commander Royal Artillery of 46th Division was wounded, and Hill Child was appointed to act in his place. A week later he was promoted to Brigadier-General and confirmed as CRA. The 46th Division had been very unlucky during the war, the infantry in particular taking appalling casualties at the Hohenzollern Redoubt and Gommecourt, but it gained revenge at the Battle of the St Quentin Canal on 29 September 1918 when it performed one of the great feats of World War I by crossing the canal and breaking open the Hindenburg Line. Careful artillery preparation and support was an integral part of this success. Hill Child had nine brigades of field artillery under his command. The bombardment began on the night of 26/27 September with harassing fire and gas shells, followed with intense bombardment with high explosive shells until the morning of the assault. Every field gun was used in carefully timed barrages: 'creeping barrages' ahead of the attacking troops, with pauses at the end of each phase, including a 'standing barrage' of three hours to allow mopping-up of the first objectives to be carried out, and the second wave of troops to pass through and renew the attack behind the creeping barrage. The first of these creeping barrages actually progressed at twice the normal pace while the infantry rushed downhill to seize the canal crossings; it was described in the Official History as 'one of the finest ever seen'. The attack was a brilliant success, and by the afternoon the field artillery batteries were crossing the canal by the bridges that had been captured or thrown across, and were coming into action on the far side. 46th Division was prominent in the pursuit of the Germans leading to the Armistice in November 1918. During the war Hill Child was Mentioned in Despatches, awarded the French Croix de Guerre, the Companionship of the Order of St Michael and St George and, in 1919, the Companionship of the Bath. He continued in the Territorial Army after the war as CRA of 46th Division from 1920 to 1924, after which he was placed in the Regular Army Reserve of Officers.
Child was appointed in 1927 Gentleman Usher in Ordinary in the Royal Household by King George V and promoted Deputy Master of the Household in 1929. He became Master in the "Year of three kings", 1936, serving King George VI until he retired from the post in 1941, but remained from 1937 Extra-Equerry to the King and, from 1952, his successor Elizabeth II. He was appointed GCVO for his personal services to the Monarch and the Royal Household in 1941, having been previously CVO in 1934 and KCVO in 1937. He also received during his service foreign honours: He inherited the baronetcy on the death of his grandfather, who had also been a Conservative MP. The title became extinct on his death in 1958, aged 78. He had made his last home at Whitton Hall in Shropshire by 1948 and was buried in the parish churchyard at nearby Westbury.
