I dont know about the "I love you"s but what i've done with the step-greatgrandsons is start a habit where we hand slap to say goodbye, a high five.  They seem to like that better than hugging, anyway.  I also say the same thing each time,  you might consider "See you later, alligator" or "Toodloo", something the kids enjoy saying which isnt so emotionally loaded for you.

Fist bumps are good too.  I like the idea of a saying.  Tigger always said Tata for now.  

Hugs and kisses on the cheeks. 
if sd's tried that on me i wouldn't allow it. Its just shake hands but i am in such a way that them trying to force hugs will look awkward being forced on me. My body language is in such a way civil hand shake is enough. Hugging and cheek kissesand for me and my 2 bio kids and my husband. Anyone else no thanks. 
 

i certainly wouldn't allow a fake kiss and hugs for photo and public show sake. Nope not happening.

in your case with skid having their kids say love you, just a polite "thanks for coming, see you again" is sufficient.

if they bring it up with your husband that you never say love you back, they aren't your bio kids, you didn't adopt them or give birth to them. You are connected via marriage to your husband so being expected to love them like your own is unrealistic.

i remember when hubbies exwife called my sil who told hubby she was telling sil that i love skids like my own. That was batshit crazy bio mum trying to falsely elevate her kids in priority status and demanding i love them like my own when they were nothing but rude, disrespectful and absolute torture to be around.

i was frank with my husband and told him me being expected to love them like my own is hypocritical considering she never wanted or loved her kids yet a complete stranger unrelated to them is expected to love them like her own from day 1? I told my husband if he wanted skids to be likeable, then them being toxic disrespectful arseholes doesn't help

I kind of feel like this is the kind of white lie that may be something to consider.  But, if you truly are nowhere near a place where you can pretend.. even for a toddler.... there are some things you could do.

1.  Arrange yourself so you are not present when they leave your home...or so that you are arranged in a place where you are not proximate to the child when you leave somewhere they are.. their home..restaurant.  The distance could prevent this "i love you showdown".

2.  You can come up with some things to say and do.. (like the fist bumps/high five's above).. and you could be on the offensive of these things.. say YOUR goodbye first.. and then when they say "I love you back?".. you may not necessarily have to say it.. or you can say.. "aren't you sweet".. or "awww how nice".. 

I mean.. I kind of get it.. my family was not huggy/kissy.. not a lot of "i love yous" not that we didn't feel loved it just wasn't as demonstrative like that.  My DH's family.. very different.  and I know I used to get my MIL telling my OSD to "give ESMOD a hug and tell her you love her".. and OSD was very much not a huggy child.. I would look at her.. she would look at me.. and we would give each other that "cringe side hug" and then I would laugh about it with her... Generally, it's really not a good thing to force kids to perform like this.. they should be able to decide if they want to do the huggy..i love yous.