Although small cells (SC) densification approach plays a prominent role in achieving the data rate and coverage requirements in 5G networks, it poses serious challenges concerning the flexible and cost efficient backhauling solutions. The traditional terrestrial backhauling hubs are subject to limited line of sight probabilities in such dense networks. Having this challenge in hand, and recognizing the increasing interest in the unmanned aerial vehicle (UAV) enabled communication systems, we address the problem of wireless multihop backhauling of SCs using UAV hubs. We present two linear optimization programs that optimize the SC-UAV association and the SC-SC formation in order to maximize the total backhaul flow. Some practical constraints are considered, such as backhaul reliability, association criteria, SC relaying capacity and the number of available links at each SC. Numerical simulations show that the approach allowing partial demand fulfillment of the SCs outperforms the binary one in terms of accumulated rate and the percentage of associated SCs, even at low number of maximum hops and links allowed.