Je les aime vraiment.

OR

Vraiment, je les aime.

Vraiment is an adverb that modifies the verb, aimer. French places adverbs after the conjugated verb. Longer adverbs can be placed in at the beginning of the phrase for emphasis.

About the placement of adverbs, laits.utexas.edu/tex/pr/adv2.html#Place says:

adverbs that modify an adjective or another adverb

Adverbs are placed directly before the adjective or adverb that they
modify.

adverbs that modify a verb

Adverbs are usually placed immediately after the conjugated verb. If
the verb is negative, the adverb is placed after the negation.

The adverb refers mostly to :

• Verbe : Je les aime vraiment. (After)
• Adjective : Le gâteau est vraiment délicieux. (Before)
• Adverbe : Vous serez vraiment mieux ainsi. (Before)
• a sentence or a proposition : Vraiment, vous n’avez pas de chance avec cette voiture. (Before)

Source : http://grammaire.reverso.net/1_1_28_Ladverbe.shtml