Don’t just say I love you.
Tell them what it means.
When a child says I love you to his / her parent I guess it means one of the following (or a mix):
#thank you
#I feel safe with you
#you give me security
#I know you don’t want to hurt me
#I know you care about me
#I know you didn’t mean to hurt me
#I’m happy
#I like to be with you (it feels nice)
#I like to play with you (it feels nice)
#I know I can count on you
#I trust you (I know I may trust you)
#you showed me that what I say or feel matter to you
#you care about my needs
#you showed empathy
#you tried to undestand
#you believe me (even though sometimes I like to make things up)
#you didn’t judge me
#you accept me
#you didn’t try to fix me
#you gave me freedom (with boundaries)
#you asked me for my opinion
#you let me make a choice
#you let me explore
#you understood that my needs might be different than your needs
#you didn’t laugh at my imperfections in order to gain popularity among your friends, family members or acquaintances
#you were honest
#you hugged me when I needed it
#you let me be alone when I needed it
#you prepared my favourite meal
#you bought me my favorite chewing gum
#you spent time with me and you were present
#you didn’t play a know-it-all
#you were flexible
#you stood up for me
#you were patient and gave me time to learn certain things.
Of course this is not a comprehensive list.
Even if the child doesn’t understand some of those words I used, that’s the feeling (we don’t need to know the word for it to be able to feel certain 
).