The key to cephalopod hatchlings and juveniles was extracted from the provisional key to young stages of all cephalopod families, presented in Sweeney et al., 1992. As these editors write in the introduction, their key is artificial and doesn't imply phylogenetic relationships by the used key characters; also, the keys were designed to identify the earliest "larval" stages, from hatchlings to animals of about 10 mm mantle length.
As for some groups not always the earliest stages are known, some key characters may fail to apply to the smaller individuals. Several species cannot be distinguished because no distinctive characters are known to separate young stages at the species level. In particular this is the case in Sepia, Loligo, Rossia, Sepietta, Sepiola and the two species that represent the Family Cranchiidae in the area.
The following chapters from Sweeney et al., 1992 were used for the present key:

¥ All families — M.J. Sweeney, C.F.E. Roper, K.M. Mangold, M.R. Clarke and
S. v. Boletzky; pp 1-19.
¥ Order Sepioidea — C.C. Lu, A. Guerra, F. Palumbo and W.C. Summers; pp 21-36.
¥ Family Loliginidae — R.T. Hanlon, S. v. Boletzky, T. Okutani, G. Perez-Gandaras,
P. Sanchez, C. Sousa-Reis and M. Vecchione; pp 37-53.
¥ Family Architeuthidae — C.F.E. Roper; pp 99-103.
¥ Family Ommastrephidae — J.H. Wormuth, R.K. O'Dor, N. Balch, M.C. Dunning,
E.C. Forch, R.T.F. Harman and T.W. Rowell; pp 105-119.
¥ Family Onychoteuthidae — M.R. Clarke; pp 127-137.
¥ Family Gonatidae — T. Okutani and M.R. Clarke; pp 139-156.
¥ Family Brachioteuthidae — C.F.E. Roper and M.J. Sweeney; pp 157-163.
¥ Family Cranchiidae — N.A. Voss, S.J. Stephen and Zh. Dong; pp 187-210.
¥ Order Octopoda — F.G. Hochberg, M. Nixon and R.B. Toll; pp 213-268.

Remark
Not all Cephalopoda species that (might) occur in the North Sea are dealt with in the key. The species below, marked with b are not incorporated because these species live benthically and their larvae are not pelagic, or likely to occur in the North Sea plankton. Species marked with a are (sub)arctic species and possible present in the Norwegian Deep and the deeper parts of the Skagerrak; but they are included in the key only if they are pelagic as adult or as juvenile.

Order Decapoda
Sepia elegans de Blainville, 1827
Sepia officinalis Linnaeus, 1758
Sepiella japonica Sasaki, 1929
Sepia orbignyana de Férussac, 1826
a Rossia glaucopis Lovén, 1845
Rossia macrosoma (Della Chiaje, 1828)
Sepietta neglecta Naef, 1916
Sepietta oweniana (d'Orbigny, 1839)
Sepiola atlantica d'Orbigny, 1839
Sepiola aurantiaca Jatta, 1896
Architeuthis dux Steenstrup, 1857
Brachioteuthis riisei (Steenstrup, 1882)
Galiteuthis armata Joubin, 1898
Teuthowenia megalops (Prosch, 1847)
Gonatus steenstrupi Kristensen, 1981
Alloteuthis medius Linnaeus, 1758
Alloteuthis subulatus (de Lamarck, 1798)
LLoligo forbesii Steenstrup, 1856
Loligo vulgaris de Lamarck, 1798
Illex coindetti (Vérany, 1839)
Ommastrephes bartramii (Lesueur, 1821)
Stenoteuthis pteropus
Todarodes sagittatus (de Lamarck, 1798)
Todaropsis eblanae (Ball, 1841)
Onychoteuthis banksii (Leach, 1871)

Order Octopoda
a Cirroteuthis muelleri Eschricht, 1838
Haliphron atlanticus Steenstrup, 1861
a b Bathypolypus arcticus Prosch, 1849]
a b Benthoctopus piscatorum (Verrill, 1879)
Eledone cirrhosa (de Lamarck, 1798)
a b Graneledona verrucosa (Verrill, 1881)
Octopus vulgaris Cuvier, 1797