Their eldest daughter is called Amalia (like the Princess of Orange - and a Nassau family name: Amalia van Solms), not Amelia.
If they continue to work with the letters a, i, l and m as they did for their first two children, there are several options - to list a few (in bold those using all four letters):
Boys: Ali, Alim, Amil, Ilai, Imma, Ilam, Lam, Lim, Mal, Mil
Girls: Alaila, Alala, Aila, Alia, Aima, Amama, Imima, Imma, Immama, Immi, Immila, Laila, Lamia, Lim, Lila, Lili, Maila, Mia, Mimi, Mila, Mili, Milla
So far:
Amalia Gabriela Maria Theresa
Liam Henri Hartmut
So, the grandparents were already included, so they might indeed go one generation further back for their third child for middle names: Charlotte for a girl (both Josephine Charlotte and reigning grand duchess Charlotte - whose husband was called Félix) and Jean for a boy (on Félix side) or they could use the names of siblings (Felicia would be a great female alternative for Félix but I doubt they use it) or include one of their own names: in Claire's case that could be her second name Margaretha (also a name in the GD family) - in Félix' case using his own name would also be a nod to his brother-in-law.
Lili/Lily/Layla Margareta Charlotte
Elias Jean Félix
And for godparents I assume Louis for Félix (although he could of course also pick Guillaume) as godfather and one of the above mentioned witnesses/friends of Claire as godmother.