You can ask LH to change the status of the child pax to CHD from ADT (hint its required to book the child meal). LH C has a few blocked seats reserved for the station to release (for example 11AC in a 343/346/333) and they should be able to accomodate. Travelling on a mixture of revenue/US DM / BD DC / M&M award tickets with my family in longhaul C last year (was burning miles for the family, requalifying my SEN and had to use the SEN WL for the fourth seat via M&M) I was spread over the cabin in pretty much every longhaul leg which got sorted out either at the gate or by pretty militant pursers, who on request, made polite yet stern requests to fellow pax to move so the family could sit together.@:-)