Photographing the kids is not exactly a job you can easily do. Often kids are a bit reluctant at first when they see a photographer with a big camera and a huge speed light on top. You try to tell them what to do and they will do the opposite right away.  You have a plan, a great game or story in mind but they hate it and are grumpy as Grouchy Smurf. For kids it is actually quite a fun to run away or look on the opposite direction. At the end of the day they just want to run around and posing for you is not their idea of fun.
You could either give up this job or actually go with the flow and enjoy. It is rather amusing believe me, you just need a few tips and a great deal of patience.
If you try to pose a child as an adult they will hate you and it will be the most boring experience in the world. Lets make it enjoyable, something kids will look forward to. Is that possible? It sure is.
What are you trying to capture. Do you want a stiff looking child in a perfect pose or you want to see their personality, smiles or even naughty faces. This is what they are and this is what will make mum and dad love this images.
Not everything will work with every family or  children. Move ahead quickly and see what will work for you this time.
1. KEEP EVERYONE CLOSE TO EACH OTHER - this will provoke some interactions and often result in surprisingly  emotional images. Ask them to touch heads and snuggle close or squish cheeks together.
2. HOLDING HANDS - ask everyone to hold hands and swing them around. This is usually a sure smile and fun and kids don't get freaked out of being close together for too long. They can even jump for even fun. Be prepared for the jump though.
3. PLAY A COPYCAT - ask everyone to copy you even parents while you do a few funny poses. This will get all involved and kids love to see mum and dad with awkward faces. Praise the kids and ask them to make a pose you like to shoot.
4. RUN - ask kids to run away from mum and dad and ask parent to try to hold/snatch kids still. Or move kids at front and ask them to tur around and run to mum and dad. Opposite will do too. This is a tricky one but with practice you can master it and get a real treasure shots.
5. FUNNY FACES - tell everyone to make an angry face, wondering face, crazy face, sad face, funny face, smiley face
6. DANCE/WIGGLE - if you ask the group to wiggle bumps and dance you'll have a huge laugh and a few great shots
7. CHANGE POSITIONS - standing, sitting, laying down. Have chars or suitable for sitting props
8. TICKLE - tell them to tickle each other and watch the fun.
9. QUESTIONS - have a word with mum and dad beforehand what kids like, dislike like favorite funny characters, movies, games etc. Ask them questions what was the funniest thing happening in this movie or book and let them tell you.
10. JOKES - learn a few good old jokes. It is useful to check for appropriate k=jokes according to kids age as they tend to change with age.
11.GET YOURSELF CLOSE - do some close ups of faces, hands or details.
12. IMPROVISE - try variations, think out of the box, keep a whistle, balloons and small treats handy.

For me it is really a good mix of everything and sensing what the mood is and going with the flow. It is very important to have your exposure  checked in advance and just move around shooting and have fun yourself. If you stop every few minutes to get your images checked or change camera settings everyone will get bored quickly and you will struggle.

See the images below. This family came all wet down from the pouring rain outside. Mum and dad din't mind carrying on with the session and here is what we ended up. Messy wet hair and the cutest little sister ever, quite determined to cuddle and kiss her brother. He was a bit older though and not in a mood of kissing but with a little encouragement and tricks we ended up with these. Love them